Images not being set to expired at the right date and I can't change it

I'm using NB 7.5.0.3 on Solaris. I have some tapes that were somehow set to expire in december 16th 2013 but they contain images that have a retention period of 2 years and the tapes were first mounted in around december 5th 2013! I tried to change the media expiration dates with bpexpdate and it just says "unable to process request". As it so happens, we did a reboot of our server this morning and it didn't change anything... Anyone ever had a similar problem?

Thanks!