Just where is the leak ?
That is the un-answered piece of the puzzle.
Have the o-rings been replaced in the hose end fitting that clips to the tubes at the heater core ?
Has a leak detector dye been used to determine the leak ?
They could be ''pissin in the wind'' replacing cores when the leak is elsewhere .
-- I've seen a pin hole leak either in a tube , hose , or fitting that sprays a hairline thin, barely visible, only under pressure stream, off to ''over there'' and the diag tends to focus on wher the DRIP is and not following back to the source
-- or --
they could be damaging each core with improper installation. I've seen it happen as the person attempts to attach the hoses , they tend to wiggle it or twist it on and THAT action cracks the weld where the tube meets the core.