Disk brakes don't work well after taking out pads and putting them back
If you set back the pistons (which is actually unnecessary if you are just inspecting the pads), pump the brake lever to move the pads back onto the rotor.

You can also clean the rotor with some solvent to make sure it is not contaminated with any grease or oil.

Update:

You can also check to see if you have a sticky piston in one side of the caliper that is not advancing properly. Unsticking them is easy, and guides on how to do so are easily found.