I would strongly recommend that you call the guy back and have if fix
the problem. If he did indeed install new hangers, he should have
repitched all your gutters to drain correct. I would start with
that, if they refuse, it is not that hard to repitch. The most
diffecult part is making sure you have a latter tall enough.
Unless you get up there to see if the angle is good from end to end you
wont know what is the truth. It could be a long gutter run and small
downspout not taking all the water or a poor job done and not enough
angle. You might need 2 downspouts to handle all the water. My gutters
were put in by a hack, bowed down in the middle and downspout 4" to
high, they held water all year.