Fitted 2 Rads one heats up quickly the other does not?

I have fitted 2 rads one upstairs and one down, the one upstairs is a ladder rad 1785 X 585 the one downstairs is a ladder 1180 X 495. The pipework has been "T" off to feed upstairs rad and then feed downstairs. The problem is the upstairs rad got hot quickly but the downstairs did not. I closed the rad upstairs and the one downstairs heated up fine.
I have now turned down the valve to the upstairs and both rads now heat up although the upstairs feels slightly warmer.
Is this the correct thing to do or should the valve be fully open? Is this a balancing issue?
Or does the pump need turning up if possible?
I would really appreciate some feedback on this issue. Thank you in advance for any help you can provide!