We recently had multiple new radiators fitted (with Danfoss RAS-C2 valves) and hit exactly the same issue. Not a single one of the radiators with HR91 units worked. The older HR80 units 'sort of' worked, but not on all radiators and not all the time.

After talking with Evohome support, they confirmed that the HR91 (and HR-92 in default mode) do NOT fully open the valve to the end stop position and so would not work as I had them fitted.

I did not want to switch out for HR92 units at significant expense, so I modified (bodged) the Evohome adaptors to work and added some basic instructions here for those brave enough to so a simple mod:

I modified 6 HR91 adaptors and only broke one of them (10 replacement cost) and every one is now working perfectly. I wonder is there is a new manufacturing process on the Danfoss valves? Our plumber said they had used this combination before with no issues...