My Howa has a "trap door" magazine and I have been inserted them as I would with loaded rounds.

I'll try one just resting on top of the "trap door" plate and I'll try loading a dummy round with bullet inserted, that possibly could be the answer as loaded rounds I've tried chamber and eject fine.

I've just googled the problem and several people have stated that in their experience cases fired from another rifle never chamber at all or not well in another rifle due to headspace etc.

It's the first time I've had this problem with "once fired" cases, and it could be totally new cases from now on instead of a mixture.