If you're getting client side errors, it's very likely that the client doesn't have all of the required certificates to build the trust chain. If, for example, you have a three-level certificate architecture
(CA -> subCA -> issued server certificates)
and the client only has the root CA certificate, when the server presents its certificate to the client, the client will not be able to build a path from that cert to its explicitly trusted CA root. You mobile clients, I'm guessing, do not have the intermediate certificate(s) installed.