FWIW short stroking or turning the turret with the index ratchet in the engaged position can ruin the ratchet and cause this problem almost immediately.
I ruined my first ratchet in the first 50 rounds.
The second lasted 300 or so.
The third lasted about 12,000.
I'm on the fourth now, with around 28,000 on the index system, and about 54,000 total on the press.
I seem to recall the intermittent problem happening when the ratchet square is upside down. There is an up and a down side.
The ratchet is nylon, and will naturally flex. They often look okay when you take them out, but do not hold shape under load. Look at the raised area on one side. Even if the ratchet looks otherwise good, once that thin area gets chewed up, you can bet it warping.
They're cheap and easy to replace.
This can also be caused by loosening of the plastic box the ratchet rides in and needs tightened. Eventually it will wear to where it can't be tightened. get a longer screw with the same threads, and put a nut on the other end.
I have ran the same index ratchet for about 15,000 rounds now and its working fine. Putting a bolt with a nut on the end to hold the clamp made a huge difference.