As a prior-service Marine now in the Army, where can I go to sort out problems with getting my Marine awards on my ERB?

1) Start with your DD214. Everything you earned Medal/Ribbon wise should be there. If it is not, you'll need to get a DD215 (or updated DD214) from HQMC reflecting your correct awards.
2) "Generally" speaking, any award that has a cross-service equivalent can be worn in any service. So your Good Conduct, and Sea Service are both valid awards, as would any NAM or NCMs you may have received. Campaign Medals, NDSM, and Unit Awards all transfer over.
3) Things that don't transfer over are "usually" service specific like Army Service Ribbon (going TO the USMC), or items that have Badges in lieu of Medal/Ribbons (like Rifle/Pistol Medals from the Navy). You'll want to check the US Army 600-8-22 and DA PAM and AR 670-1 (Ch22).

The confusing part will be signing the order of Precedence. Your Marine/Navy achievement or Commendation Medal and your Army Commendation Medal should be side by side but I don't know what goes last and what goes righr?

Did this ever get sorted out? I'm having the same issue after transitioning to the NG. The DD214 isn't a valid source document for awards. The original award citations are. There are no award citations for sea service deployment ribbons and afghan campaign medals and such, so what is the appropriate source document?

O how times change. In 1963 I joined the army after serving 4years in the Marine corp. I went from Corporal in the Marine Corp to PFC in the Army. It was a change I couldn't believe how easy it was in the Army. 15 months later I was a SP/5. I didn't have to worry about my medals. All I had was a Marine Good conduct medal. I also had an excellent shooters badge. Today They have to many medals for nothing. When I got discharged in 1966 , I had a Marine and Army good conduct service medal, Vietnam service medal, Vietnam campaign medal. Vietnam cross of gallantry metal and a National Defense Medal. The Vietnam campaign and the cross of gallantry medal was awarded to every GI< that served in Vietnam. Just before I left Vietnam I was offered a Battlefield Commission as a 2nd Lt. if I stayed in Vietnam another year. I had only been married 8 months I turned it down and returned to civilian life where I worked for GM, for 35 yrs. Semper Fi.

Your ERB is just a "brief" of your OMPF (iPERMS). To wear an award, the award must be in your OPMF (iPERMS). If you can see your award in your iPERMS, and it is authorized for wear in AR and DA PAM 670-1, you are authorized to wear it.
If the document is not in iPERMS, provide it to your BN S1 and ask them to batch it to iPERMS.
Order of precedence, including awards from other branches can be found in DA PAM 670-1starting at Ch 20-5.

LTC Jason Mackay had a recent post that had all the details spelled out with enough specificity to get your awards recognized with very little pain.
I also always go with the Army as first in order of precedence being it's the oldest service.

I have 4 years in the Corps as well and I’ve been Army for about 7 years now. My awards have still not shown up. The required documentation requested by the S1 OIC was my 214, and since I’ve had awards given after Corps service, any documentation proving I was indeed awarded them. If you can’t find these docs, try IPERMS. I would suggest giving it 3-6 months for your ERB to update before you do anything drastic such as going to a higher echelon. Even then, try and work with your BN S-1 as much as possible. I recently went through the paperwork to get my enlisted years added to my O1 DOR and that process took about 9 months. Just keep trying and be patient.

I am really surprised that your S1 is on the their ass in this problem. It's kinda their job to walk you through and get it straightened out. I'm sure you have a DD214 + orders awarding them.
If a soldier is supposed to receive a personnel award or even a unit award because he served during a particular time frame, Army General Orders will list who, what, when, where and time frame awarded.
I'm pretty sure the \Navy holds the same for the USMC. Maybe your S1 is searching wrong data base.

Ok so this is hard because it’s a lot of paperwork and looking up awards S1 don’t want to do their job. It’s took me a whiles but I get some of my stuff in. So it’s easy if you take the Navy route than Marines it’s hard to find your awards under Marines but it’s easy to find it in the Navy section that how I was able to transfer most of my stuff. The only things I’m trying to transfer now is my MCMAP for Army Combatives Since it’s worth promotion points.
Marines Deployment on my ERB letter of appreciation which is the same as COA.
The date of Rank is always going to get jack up
Because the day you became active that your new date of rank.

I had this problem when I joined the Army. I had my Marine Corps DD214 with my awards and just had the SPC at S1 add my ribbons to my ERB. The issue I had was I was awarded ribbons after the Gulf War that were not on my DD214. I went online and requested my updated ribbons and they emailed my the official records update. I again took that to S1 and updated my awards. None the order of my Marine/Navy/Army awards. I just went on Rack Builder online and entered all my ribbons and clicked the Army tab. Rack builder put my awards in order to Army Regs. Between all three branches I ended up with 23 ribbons and had Rack Builder send me my rack all put together. The funniest thing was my brand new LT thought my marine Corps expeditionary ribbon was a Bronze Star.