Edit: beaten to the punch by others..

Make sure you've adjusted the b-limit screw on the RD. If you've got a short cage RD, it may not technically be able to take a 28-tooth cog, but it should be close enough where it would work fine. I've got an Ultegra 6600 rear RD that's supposed to only work with up to 27-tooth cogs and got it working with a 28-tooth cog by properly adjusting the b-limit screw. One tooth beyond the stated max capacity should be fine even for a short cage RD if it's properly adjusted.