(ansys) why can't I get contact result if bulk material is defined by usermat?

hi everyone:

I  tried to perform a cohesive zone analyse in ANSYS, followed the turorial in HELP file. I used contact element to model cohesive zone. I found out that:

1, If I use standard material of ansys to define bulk element, like defined by 'mp,ex,1,2.1d5' or 'tb,biso', I can get a reasonable contact result;

2, If I use usermat file to define bulk element, 'tb,user', then the solusion also is done, but the resulte seems not correct, two part of model are not contact, some sections of them are coincide.

Are there any special settings on using usermat in contact analyze?

Thank you all~~