Most likely suspect is cracked/worn collet nuts (aka collet nose).

9/72 model collets were the "new" cast style (right side). As mentioned, they have a tendency to crack from tightening. Replacing with a machined collet (left side), Rogers or newer repo made, should help in that case.

If you use a drill stop collar (with allen head or square head drum screw) on the legs as a Memriloc, you do not need to tighten the collets as much. Looks better than a hose clamp as well.

Thanks for the replies. So If I understand you correctly, the cast collets are softer and not as durable and can't "grip" as tight as the machined collets?

I use the shaft collars on the insides of the spurs, so they automatically extend out to the correct length. I guess I could use them on the outside as well. I just went with the hose clamp because I didn't have to take the head off and lose my positioning on the inside, as you can attach a hose clamp from the outside (it comes apart).

Man, those machined collet noses are expensive!

There can be multiple things at play here.

As was stated the Machined collet noses cracked under high tension and were basically CBS trash.

Also sometimes the fingers stretch out... going inside with needle nose pliers and stretching them outward slightly helps.

Yes seems odd to stretch them outward but it gives the nose a better grip on the threads.

Lastly what most don't realize is that the size of the hex stock in the Ohio era was different than the hex stock used in the California era ... this is often the most common problem when people seek out missing hex parts to replace or switch out collet plates

Your parts may simply be from different eras.

You have it backwards. The Machined ones were the good ones. The cast ones were junk.

I mistyped

yes the cast were garbage... what I meant