You'd look in the MAGT log.
You can also use the messageflow.exe tool to get a better picture of what's going on.
BES Logs ... What's in them? Part 19 of 20 (MessageFlow.exe BRK Tool) - Port3101.org : Your BES Connection
Do the devices have good wireless data coverage?
Are you sure the data plan is still provisioned on the users' accounts?
Can the devices receive PIN messages?