Loss of Midi Control

I am using Multiplay to control my mixer scenes. Recently I changed mixer to Allen & Heath QU5 which uses software on the computer (Allen & Heath Midi Control) rather than external hardware to output the midi control to the mixer. All works well until suddenly it stops working. I have found that editing in Multiplay can cause this, but it also happens whilst running a show. The disconnection is not reflected in Multiplay ie. it doesn’t freeze like it does if you pull out the hardware USB. Any ideas?
Brian