Joined: Feb 2012
Posts: 19,353
Likes: 5,471
From: Rochester, NY
Bikes: Stewart S&S coupled sport tourer, Stewart Sunday light, Stewart Commuting, Stewart Touring, Co Motion Tandem, Stewart 3-Spd, Stewart Track, Fuji Finest, Mongoose Tomac ATB, GT Bravado ATB, JCP Folder, Stewart 650B ATB
There are very slight differences in pin/link width between the various chains of the same "speed" spec. But these differences are in the order of 0.1mm or so (about 4 thousandths of an inch).
You could try a different connection link. The same brand as the yet to be known chain. The chain and cog could be so worn that only an old worn link will run skip free. The connecting link might not be fully snapped closed (as mentioned already by Wanderer). You could have a bent tooth. You could have a missing cog spacer (both of these are doubtful though). Andy.