gas valve
I fully agree with the cleaning advice, the bolt and carrier group, the transfer piston and the gas system could all be contributing by causing friction and robbing power.
I've had two cases where a Yugo was failing to eject/load, both times I ended up buying new gas valves and installing, as well as getting a gasport tool and reaming out the port really good (a small drill bit will work, too, just do it by hand). The valves had been corroded badly and I think enough leak by was occuring that it caused the rifles to malfunction. In both cases there was not a apparent gas leak when fired.
There's also some info on the web of guys who've used teflon tape, O-rings and copper wire crush rings to seal their rifles up, haven't tried these myself and would recommend doing it the way you know is going to work, by replacing.
Ebay had one gas valve I ordered, I don't recall where i got the first, any web search should lead you to one, however.
Good luck... my Yugo is a great shooter now that she's right.