Ok guys I figured out what the problem was (and it was really stupid - I really feel quite retarded right now).

My main account uses X-Perl, and when I was casting one spell, then spam casting a different one, the cast bar on my main character's frame would say "FAILED!" in red. I found out the spell is actually still casting (spread my characters out and tested). In X-Perl pre-3.0 it didn't do this, it would just keep showing your existing casting bar without putting the FAILED message up, not sure why this changed - hopefully its just a config option I can change.