Was the cluster reboot done after applying the device pack on all the servers?
Can you try restarting the TFTP service on all the TFTP servers as well.
What is the exact message on the display of 8941 IP phones?
Are all of these phones remote to the TFTP server or even the phones local to the TFTP server facing the issue?
If u are using CUCM version 9.1(2) with 9.4(1) for 8941 , u require a Device pack for the phone to work.Even though it was working earlier, can u check for Device Pack?also, share show version active
Also, cross-check as Brian has suggested.You can login through CLI and run file list tftp *8941* to check the available firmwares.