We eventually got to the bottom of it. The plastic covers are supposed to bear against the transistors underneath and hold them tight against the heatsink assembly. But it's a really unsatisfactory approach, because they're not secured at the bottom in any way, so the devices are not pressed evenly against their pads. I'm looking into whether it can be improved whilst waiting for the spares to arrive. Perhaps I can get some of those clips that Whit3rd posted a link to.