Getting looked at for SFC this year. What are the top reasons for not getting selected? Any Advice?

Have secondary skill specialties (plus others if you are able to) as well as leadership positions while overseas

Bad NCOER, PT failure, NJP, banging the CSMs wife to name a few.

NCOERs that are written poorly, one line bullets with no real details. Senior rater bullets are very important so, Please always pay attention to the comments.

Here is the link for last years board results (AAR, stats, and list). Read it!!
https://www.hrc.army.mil/content/FY17%20SFC%20AC%20Promotion%20List

Here is the order of getting your records straight from my exp:
1. Check your SRB/ERB, iPerms/OMPF for any missing or out dated items (college transcripts, certifications, training, awards)
2. Make an Appt with your S1 to get your SRB/ERB is up-to-date and everything that can be put in your iPERMS/OMPF is and matches your SRB/ERB.
3. Get a fresh hair cut and new DA Photo (ensure your uniform matches your SRB/ERB) this is your first impression to the board. Have a few Sr NCOs inspect you in your uniform prior to taking the photo and have one with you when you take the photo to ensure you look your best!!
4. Validate your online board file once you have everything up-to-date and prior to the deadline!!

Good Luck, God Bless!