A gutter that overflows in heavy rain is not necessarily blocked, and it is not necessarily too small either. More often the downpipes cannot clear it fast enough.
Every gutter run drains a certain area of roof. That catchment area, along with the rainfall intensity the gutter has to cope with, sets both the gutter profile and the number and size of downpipes it needs. Get the gutter right and the downpipes wrong and the gutter still fills faster than it empties.
This shows up most on additions. A patio or a room extension adds roof area feeding into a gutter run that was sized for the original house, and nobody adds an outlet. The gutter was fine for twenty years and now it overflows every winter, which makes it look like a blockage.
The other half of the problem is fall. A gutter run needs a deliberate fall toward its outlets. A dead-flat run holds water after every shower, and standing water is what rusts a gutter from the inside out.
If your gutter overflows in the same spot every time, count the downpipes on that run before you buy a longer ladder.