Jimro, thanks for chipping in. I just went and tried to do both: seat the bullets all the way forward, and right up to the back. With the bullets forward I could actually strip and engage up to 3 rounds. The 4th round wouldn't strip at all - it looked like the 243 shoulders were too tightly pressed against each other with the 4th one in.
Then I tried to have them all to the back. Well, with the 1 round I sometimes could strip and engage, but not always. Then I tried to load more, working up to the 4th round: all seated way back. Nope. Worst - from round #2 onwards it just wouldn't strip nor engage.
Erm, what's a magazine block?
Is it any problem with the bolt at all??