same dongle works perfectly on Daphile running on a thin client PC in the same position
Assuming you mean exactly the same dongle and not another dongle that has the same branding etc ... then that implies that there might be a setting or driver function that could be changed/enabled that would make it work better with a weak signal than the default in pCP.
The challenge would be to find what that is and then hope that it can be tweaked in the driver that pCP supplies (or find a different driver).