Sounds like glazing or poor quality materials. In any case, take the pads out again, wire brush them and give the discs a squirt of brake cleaner then dry them off with clean kitchen roll. Make sure there is nothing causing the pads not to come off the disc when you aren't braking i.e. sticky pad guides or the plungers that hold both parts of the caliper together. They are prone to that. Uneven pad wear is also a syptom of sticking. Does it only happen when braking or when travelling slowly?