If the gutter is tilting away from your house it could be causing an overflow particularly during heavy storms.
During heavy rain water jumpa over the gutter.
This valley shoots rain in a flume during our frequently heavy downpours.
Too few downspouts will tend to make the gutters overflow during a heavy rain.
With surface tension covers leaves slip off the edge while water moves into the gutters.
Foam or sponge based inserts go into your gutters and allow water to trickle through the foam into the gutter and downspout.
First your gutters may be clogged with leaves.
Currently there is a diverter moved a little way up the valley and black sponge intended for putting inside a gutter has been wired onto the mess guard on top of the gutter.
Holes through the splash guard and the gutter.
You can find these precut gutter splash guards about 3 in brown and white aluminum at a home center but you could easily make your own out of aluminum or sheet metal and spray paint them to match your gutters.
Surface tension gutter covers conquer heavy rain.
A 5 inch gutter is typical but 6 inch gutters are also available.
Same goes for lack of pitch which causes the water to gather in one spot rather than flowing toward an exit.
The pitch of gutter heading towards a downspout can also cause overflow if it s too steep.
Next is the size of the gutter itself.
However during heavy rain water may not move quickly enough through the foam.
The second thing that causes this is either insufficiently sized downspouts for the run or else a clogged downspout often up near the top of the downspout but still in the gutter.
Drill rivet holes for the gutter splash guard.
Think of it this way.