I had that problem not too long ago after a new hire came on. ACL push was being run successfully against all my windows targets, but our Unix admin did not notice that the ACL push had stopped running against the appserver/fileserver. We did ACL push to the file server and that solved the problem.
actually - it could also be that you did push ACLs to the file server and that's causing the problem.
typically we don't reccommend pushing ACLs to the file server - set a mapping in the exports file, and leave the users and users.local blank.