Arpeggiator doesn't work with MIDI controllers?

Hello,

I have an iPhone 5s with the latest iOS and Garage Band.

I am connecting a Kieth McMillan 12 step MIDI foot controller to my iPhone to play Garage Band sounds using a camera connection adapter.

Sounds come out fine but, if I turn on the arpeggiator, the controller does not trigger notes with the arpeggiator; it just plays the sounds as if the arpeggiator was off. If I touch the screen to play on scree keys, the arpeggiator is working fine.

Does GarageBand only allow for the arpeggiator to work if you play the keys on the on-screen keyboard only and does not support external MIDI controllers? Is that what's happening? or is it my particular controller/set up?

Please help!

Much thanks!