Yeah Ive certainly seen this problem. If you plug any synth into say a computer and it wants to download a driver, its not class compliant and it wont work on iOS. I have a couple of Roland UM-One adaptors. They are built for both PC and iPad. They are class compliant (no drivers needed) and they can be seen on the ipad.
Rolands Aria range are notorious for this, they all have USB outs but none of them can be seen by iOS as they arent class compliant. Bit of a pain actually. But then again, they also transport Audio along with midi across the USB bus, so they are very proprietary.
Cheers