Recording Arp & drum tracks

[Col66]

I'm having problems recording the Arp & drum track MIDI in Reaper. I can record the MIDI of single notes played by myself with the keys but when I select Arp & Drum track, plus latch, and play a note to trigger the Arp and Drum track it doesn't work properly. The drum track plays fine but the Arp seems to have a long delay. When I try to record the Arp & drum track MIDI data it records nothing.
I'm assuming there are some settings that I need to sort out but I can't find out what they are.

Any help much appreciated!

Col.