What advice do you have for pursuing a re-enlistment bonus that has not been paid and is now past the 120-day mark?

Use your chain of command and give them a chance to fix it. Once that has been completed and youve given them satisfactory time to action the issue. Go to IG, funfact all IG numbers are just a google search away for any command. They will verify the correct processes have been followed and apply pressure to the appropriate stoppage in the process.

Get your congressman / congresswoman involved. You’ll see how it get fixed. Had a Soldier (while a company commander) who had a similar issue. After going thru various admin inquires and no results, I suggest him to contact his congressman. He did and the issue, by means of the Holly Spirit, got fix. Of course, I already got my reply letter already written. Like I stated, I recommended to my Soldier to write to his congressman regarding the issue after exhausting all local resources.

Who have you been working with? Does your 1SG know? If they all know and have not been helping I would contact the BN retention NCO. He/She will know if there is any kind of issues going on with the system that handles bonuses. My BN retention NCO is very good at pointing out problems with bonuses before they became emergencies. If that doesn't work talk to your BN CSM and see if he/she can get it fixed. Once you have gone that far I would keep running it up the chain but then start working things like a Congressman. Bonuses not getting paid on time is pretty common issue (even though it shouldn't be) and pinging the chain of command on a regular basis should help get things fixed. Each situation is different, so printout all of you supporting document and stay diligent. It will take time but everything will work out if you stay engaged.