How do I help my Mother recover unpaid treasury benefits?

I'm wondering also if it's appropriate to contact local law enforcement and ask them for any other advice on tracking the missing checks, assuming, but hopefully not, that there was some misappropriation involved.

I'm assuming there was no notification prior to the cessation of the deposits. I would ask the bank ASAP if there's any indication of hacking of their accounts, or any indication of someone having provided alternate information that would divert the funds elsewhere. Then contact the federal agencies first thing Monday morning.

Someone else posted in the last month or so that something similar had happened - a meddler (neighbor or relative, I don't remember which) fraudulently executed documentation to have someone else's funds transferred to her account. I don't remember all the details but I remember the bank had apparently followed this person's instructions.

Was it the facility where she lives that brought this to your attention?

Checks were previously deposited electronically into her checking account.

Are they deposited directly to the facility or to your mother's checking account? If the former, has the facility received any notification why the checks have stopped? If the latter, have you called either SS or SSI local or HQ offices?