Got a job a week or so ago "gutters overflowing" after a ridiculously heavy downpour.
No problem, nipped up the ladder - only nothing in the gutter, no blockage in down pipe bend. One join looked a bit iffy, so I remade & sealed it. Couldn't find anything wrong.
Got another call from the customer today, after another heavy downpour.
Back up the ladder, no change, still all clear.
Concluded that the down pipe must be blocked lower down, possibly where it entered an earthenware socket leading to a soak away.
It certainly was :-)