One thought - when barring with the first finger, roll it over slightly so that you are using the side of the finger rather than the fleshy bit (ie so that your finger nail is facing the nut a bit).
Some questions:
If you form the barre with just the first finger and forget the other fingers do you still get the problem?
Are you playing an acoustic or an electric?
Have a look at the action on your guitar. Is it high, can it be lowered? Are the strings very stiff, what guage of strings are you using?
Can you play the chord shape you described higher up the neck without a problem?
More questions than answers I'm afraid but I hope it helps a bit.
