I have 2 sets of iPad 2's. 1 set of 15 and 1 set of 26. I use these for students. The set of 15 work perfectly fine with a video dongle attached to send a single to a mirrored device, i.e.: monitor or projector. The set of 26 will not work with a dongle. There must be a setting which is different, one set from the other, but I cannot find the setting which does not match. Any suggestions as to what setting needs to be adjusted for the dongle to be compatible and functional?