1. You pushed the collet die closed all the way without a case in the die and the collet is jammed and is no longer free floating. You will need to dissemble the die and release the collet. (Over tightened die)

2. You adjusted the collet die too far down or too deeply and you buckled and deformed the shoulder making it hard to chamber. (Over tightened die)

NOTE: When all else fails read the directions for your collet die setup as this is explained very well in the directions.

"DO NOT" full length resize your .303 cases, the Enfield military chamber is 1/8 of an inch longer or deeper than civilian commercial SAAMI reloading dies. Therefore you will need to back off the FL die approximately 1/8th of an inch from the shell holder. Back the die off more than this and screw it down a little at a time until the case chambers with very little force on the bolt. (Bump the shoulder back only .001 -.002 instead of an 1/8th of an inch)

Below, the amount the case is sticking above this case gauge is how much longer the military chamber is than a civilian commercial SAAMI chamber. When you full length resize the case you are pushing the shoulder of the case back too far and shorting case life.