Thanks for your suggestions.
My method was to pinch the cylinder firmly, then slip the cap on at an angle at different contact points, searching for the best spot, then mashing down the opposite side. (That sentence could be misconstrued!) Alternatively, and in desperation, I would try to work the cap on around the perimeter like a Tupperware container. But it seemed that some cartridges were slightly malformed, being very hard to snap a cap onto.
I'll rotate the caps in the future. I was wondering if there was some kind of manufacturing inconsistency or whether it was my own inexperience: some caps snapped on easily, others took minutes of struggle. I suspect it was me.