Then it may be full of sludge, badly installed, etc., difficult to diagnose over a forum. I think you need to engage someone competent to physically come out and inspect it.
I have a one pipe system in my home (large 1960's 3 bed detached, 9 radiators, plus 2 two towel warmers) and had planned to replace it with two pipe after I moved in, I only had time to convert the first 3 radiators to 2 pipe before the first winter came but not the others. However, it worked so well that after the first winter I left the rest on the one pipe and they are still there now, 21 years later. The only difference I notice is that the radiators on the one pipe section take a bit longer to heat up from cold, but they do get hot, maybe not quite as hot as the two pipe rads, but pretty close.