Different tool manufactures use different hook & loop systems for their backing plates, not all will be 100% compatible with every make of pad.
An example would be Meguiar's, which has a Velcro for optimal use with their MF pads...3rd party backing plates can generate more heat, not have enough grip, or in some cases too much grip.
Obviously it would be best to pair your pads from the same manufacture as the backing plat manufacturer, but everyone likes to try different pads so this in rarely the case.
Also, pad over-saturation, where polish seeps down on to the backing plate, can reduce the hold of the Velcro. If your backing plate has dried polish residue on it that could be a factor as well.