The first thing we should look at is to confirm that the gutter is not sagging, or draining away from the gutter downspout.
Use a builders bubble level and check the direction of flow.
While up on the ladder, check the gutter dropouts for any hidden obstructions that may be slowing the water flow.
If everything is found to be in order, you may need additional downspouts to handle the volume of water coming off the roof.
Additional dropouts and downspouts can easily be added to existing gutter systems.