That sounds like a Zebra.  I use 4 scopes-each with there own rings and mount plates--and each has the same issue.  I think its related to PA to tell the truth.  I can routinely take 30 min subs and my stars are small and round--so I am not keen on starting to add shims and change things like that.  I should do a gnats behind drift PA and that will answer the question--but that's beyond my ability and I don't want to lose the imaging time