You obviously bought the Olympus compatible version & you are using forced flash on the camera (single lightening bolt) & RC mode is turned OFF in the camera for hotshoe operation? The Flash is set for Auto TTL as well?
I don't know what era you bought your E-M5. I bought mine about the time they were actually becoming reasonably well available, probably July of 2012. Had a FL36 that worked fine with my E620 but not with my E-M5. The little flash that comes with the camera is fired through the accessory port. My hot shoe and several parts needed to be replaced. I think this was a pretty general issue with bodies produced at that time. I'd at least try it with another strobe and make sure you don't have your camera set for RC mode.