Simple test, pull they connector off the monovalve and try your heater in all the positions.
If you don't get heat check your aux water pump, power it if need be. If you still don't get heat you probably have the diaphram problem discussed above. With the connector off you should get full heat, if not you've got either a bad valve or a restriction.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician