Have the non heating up rads have thermostatic rad valves (trv) fitted?, if yes then try to unscrew the trv head only, and push the little pin which is now exposed on the trv body in and out GENTLY, wait a couple of seconds and the pipe leading to the rads may start to heat up. The trv may be stuck and might need some gentle encouragement with a hammer to the the body of the trv to free it!

If not, the system may need balancing?