Pullmans and Kadees again!!!!!!!!!!

I know this has been done to death but can someone help, As I have some sharp curves in the fiddle yard, I need to convert the bogie rather than the buffer beam area. I've tired drilling out #20's and attaching them where the old couplers were, they are at the right height but still snag on the buffers etc, also I am still getting derailments where all else is good. I have been sent the upgrades from Hornby, but no avail. Also, the bogies seem to be sticking, it seems where the cam rubs against the back of the buffer beam Thanks Rob