You might have to turn the ax8 off and back on a few times - can be temperamental.Ive just picked up one of these Zoom U44s in gear4music sale (£80). I can get it to work with AX8 SPDIF in standalone mode (so I can use it as a headphone amp), but cant get SPDIF sync when its working as either a PC or ipad soundcard. I suspect its a bit rate issue (48k AX8 vs 44k PC and ipad default). What are you setting on your PC to get the AX8 SPDIF and PC both working?
I set reaper to 48k to match the ax8.