This problem always occurs with towel rails! They often come with 2 manual valves and different members of the household try and turn rail on by operating either and/or both valves... Usually ends up with them both fully open, rail gets very hot and one or more other rads are robbed of flow!
Best thing to do is close one valve on all rads with other fully open, put CH on, open the valve you closed (on each rad) and feel tails to see which one gets warm first to determine flow side, then quickly close and do same with next rad and so on.
Then make a list of all rads, noting flow side for each.
Close all the return valves and open all the flow valves.
Then go back round and slightly open each determined return valve and wait a few mins until you feel heat coming in to flow side, do same for each other rad.
Give it 10 mins and go back and see which rads have stalled!
Open returns on these until you again feel heat flowing through.
Repeat after 10 mins if necessary, and so on until all rads are hot as you'd expect.
Finally, go to each rad and count how many turns it takes to close the return valves (down to a quarter turn). Then open it back up same number of. 25 turns.
Add these number of turns to your rad list and keep it in a safe place for future reference!