Not hugely knowledgeable in this area, but two possibilities based on my own investigations:
- Gunk blocking warm water
- Radiators need balancing
For the gunk, best thing to do is turn off all other radiators and turn on heating, try to get hot water to flush it through.
Doing that will also answer your second question - if the rad gets very hot, its possible the rad is one of the last ones on the circuit and isnt getting enough hot water from the boiler, and the last heat is just cooling once it gets inside. Balancing is done via the front valves on the rads, not by the manual TRVs you turn your rads up and down on. Prob best to get a plumber who know what theyre doing to have a go at that