Never had a problem with My CZ550 in 416 Rigby excepy the Woodleigh Hydros. Originaly not a problem then a slight hitch with the round on the left when bolt worked slowly. Changed back to original seating depth and slick as.

What rounds are they using? Working the bolt slow or fast? Is it with just one rifle or several? What age/how long have the owners had them? What magazine box do they have, the 375H&H or 458Lott. (3down or 4 down in the mag).

Check where the front of the mag box fits to the feed ramp. Check the mag floor plate is the right way round, same with the mag spring.