drainage pipe

im trying to join corrigated (black pipe) to sewer 4" pvc. everything fits but the joint between to 2 pipe keeps leaking. i know that they have a push on fitting,but it seems to not want to close tight enough to stop leaking. is there something im missing. I have a connector that fits between the two pipes and one fits tight to the pvc, but not so tight to the black pipe,so whats wrong?