Stupid question, but have you bled the new radiator fully and opened both valves?
If you have, you may want to try balancing your system a little- throttle the lock shield on the rads that heat up quickest. This should help push water through the rads that have more resistance to flow. (This is not proper balancing, but may help without getting too complicated).
Depending on what type of system you have, you might want to check the pump speed. If it's possible to turn it up, then try it; but watch out for any adverse effects.
If you don't have any joy with the above, try returning the pipe size back to 15mm- if you have a longish run of 10mm already taken off a branch at the end of a circuit and you have fitted a large radiator to boot, then you probably aren't getting enough KW's to the rad.
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