Yes, even with a bi-directional valve, the other end will be hotter because it's the "flow" side.
Is your pipework minibore/microbore (usually 8mm or 10mm diameter) or the more conventional 15mm.
Smaller bores can bung up more easily.
A good flush through the whole system wouldn't hurt. At least you could rule that out. Re-fill using an inhibitor as well.
Another cause could be the circulating pump. Sometimes, increasing the size/capacity of the rads could necessitate upping the pump speed. It all depends on how the system was laid out in the first place. A poorly designed system, for various reasons, can often require a faster pump speed.
Having said all that, if the system hasn't been extended or increased in capacity, and it was ok before, then there's not much left to try.
Stuck-down pins on the TRV valves is not uncommon too.