Crosslinking is always tricky... often the antibodies' binding site is crosslinked when trying to crosslink the antibody to the bead. This can completely alter the specificity of the antibody for its target... and you never know what's going to happen when you crosslink.
There is a pretty new kit from Invitrogen made with dynabeads too..
Dynabeads Antibody Coupling Kit (14311D)... you get covalent antibody coupling to the beads and it's easy to use.. The background is lower than with protein A & G too.