I could not agree more Peter. I put up with an unreliable last round feed from my No.4 for years - due to being bone idle.
One day I sat down (as advised above and on umpteen other threads) and watched it carefully. The last round was getting hung up on the lip under the breech due to magazine not seating correctly. That fault was easily fixed because it was caused by a magazine lip not allowing the magazine go go home fully. Having fixed that, the tiniest of lip tweaks got the rifle feeding reliably - which it has done ever since.
The lessons learned: 1. Try feeding a drill round and WATCH CAREFULLY what actually happens. 2. Only remove the magazine for cleaning, those lips aren't made to suffer the wear and tear of being changed like a modern rifle or left unprotected out of the magazine well.