Probably because they block up with crud off the roof, when they are
empty you should take the
opportunity to flush and clean them out thoroughly, it's surprising how
fast the crud builds up.
Don

Enlarge the cut out at the top. Insert an old sieve - if it has a couple
of loops to support it, they can even be bent down so the sieve is level
and feed the incoming water through it. It will catch most of the debris
off the roof and can easily be cleaned out every few weeks.

Andy C