Who is your cert issued by? It is possible that clientside SSL profile on the BigIP is not chained correctly - if your certificate requires an intermediate certificate to be chained, some browsers/devices will give you untrusted warning because they don't have the right CA bundle/chain installed on them. I've seen issues where Windows devices would not complain of SSL, but mobile devices such as iPads would - thus I highly recommend verifying proper CA/Chain installed on the BigIP and the clientsideSSL profile.