Hi,
A few things to check:
1. Can you try replacing one of the 6921's with a different phone model and see if the issue still persists?
2. Are the 6921's local to the cucm / tftp servers or located over a WAN link?
3. Do you see the CPU / RAM / Disk usage as normal on cucm when you do a "show status" on the servers?
4. Do you see any locales being downloaded when the phone is logging in?
These can provide some hints on whether the issue is with the phone , cucm or the network.
HTH
Manish