Strange speed and no load

I’m using a Computrainer and am grateful it even works lol!  On regular rides it works as you’d expect.  But if you’re riding a workout, there’s no load.  In addition, the speed is way off.  I’ll be on my last cog ripping (because there’s no load) and the speed will be like 16kph!  Go back to a normal ride and works as usual.  Any ideas?