some of the aftermarket disc brake pads have paint or excess material on the backing plate of the pad, that stops the pad spring fitting properly onto the brake pads and stopping you from making the “pad sandwich” Avid talk about in their literature
you will see the pad spring start to “flare” when trying to fit to these pads, only when cleaned off, will the pad spring sit flat alongside each pad
take a flat needle file and file off any excess paint or material so the pad spring fits correctly onto each brake pad
push your pistons home with the ring end of a 10mm ring spanner, if you cannot get them to seat you need to reduce fluid volume; open the bleed port on the MC (brake lever) and then push the pistons flat each side with the ring spanner, you will see a little DOT leak out of the port
reinstall the bleed port on the MC, you should have no trouble fitting the pads, but watch out for the caliper’s pad spring plate, as this can move when refiting pads
often it helps to firmly grip the new pads’ mounting tab (the bit that sticks out the top of the caliper) with pliers and wiggle / SNAP the brake pad into place, you will hear it click home once located
if you cannot get this snap, you need to reset the pistons flat into the caliper body, or reposition hte caliper pad spring plate, as this often moves when messing around with the brake setup!