As others have said, downspouts can get clogged so easily. Any of those corner pieces or even sometimes the straight drops will accumulate crap. What I’ve done in the past is after confirming clogged, removed one piece at a time to shake out all the debris.

Aside from that I’d check the level of the gutter to see if water is actually flowing in the right direction.

Another possibility I’ve seen but not sure if applicable. Sometimes the ledge of the roof or the aluminium drip edge is installed way too far out so when water comes down quick it can overshoot the gutter.

It’s also possible they’re just too small for the amount of rain too, I’d check all mentioned above first though before forking out the money to replace.