Look close at your pistons. Are they dirty? Even if you clean off
the part you can see, there will still be contamination on the
pistons beyond the point where they disappear into the caliper.
Here's where I cleaned my pistons and then extended them
some more to show the "hidden" contamination:

I think jambing dirty pistons back into your calipers when
installing new pads is asking for trouble down the road.
I've heard people talk about a "short cut" way to clean
the pistons pretty well without removing anything, but
I've never done it and would probably end pushing them
too far out and making a giant mess... remove the
old pads and press on the brake pedal "the right amount"
and cause the pistons to come out far enough to clean
all the "hidden" contamination. Substituting some
thinner object for the old pads to keep the pistons
from coming out too far might help... like the pieces
of wood in my picture...
In my picture the caliper is off the bike, the fluid is gone
and I pushed the pistons out with compressed air, so my
picture is only useful for showing the hidden contamination.
Good luck...
-Mike