Uneven heat distribution is a common problem - especially in old buildings.

A common cause - easily remedied - is defective or improperly located air vents or steam traps.

Sometimes radiators have been changed and the new ones are too small.

Improper location of the indoor sensor can cause improper heat cycle time.

Of course, old windows may allow cold drafts that cause existing radiators to be inadequate.

There are many possible reasons but the above are very common and I have seen all of them in buildings I inspected.