Stmc93

Hi, looking for some advice. I have 10mm pipe feeding my radiators but its not feeding enough water into them as they don't produce as much heat as they should and have some cold spots. I don't think the pipes are blocked up because I have flushed them out with cleaners and removed and cleaned a couple of the radiators out and still not enough heat. I fitted a new radiator which also doesn't produce enough heat although it is a bigger one than the others. I have 22mm mains pipes coming off the boiler which I'm guessing reduces to 10mm under the floorboards upstairs at some point to feed the radiators. (Boiler is in an upstairs bedroom) My question is, can I cut the 10mm pipe at a point before the radiator and attach a 10/15mm reducer and feed the radiator with 15mm pipe? Would this work or would I have to lift the floorboards and replace all of the 10mm pipe completely for 15mm for it to work? Forgive me if its a stupid question, I've not long moved into my own house and still learning bits and bobs ?. Any help appreciated, cheers.