possibly the O-ring has dried out if it is an older, vintage ferrule. perhaps replace with a newer O-ring. absolute,very last resort would be fooling with male/female id/od. how old is ferruled rod?

If you install a new Oring, get a couple sizes, and get em smaller than you think you need.

Let us know what works

Not all O-rings are alike. There are three measurements to consider: OD (outside diameter) ID (inside diameter) and radial CS (cross section) which can be effected by stretching.

I have feeling the CS on your O-ring is too fat and most likely not original. A little searching online for another size should resolve your issue. You might want to try and measure the existing O-ring to at least have a starting point or just jump in a buy a bunch of sizes and try your luck.

There are lots of O-ring size charts on the Internet which may help.

I have an old Herter's rod with the o-ring'd ferrules. Mine is OK but I suspect that considering the concept, a 50 year old O-ring might not be within spec for "tolerance". If you have an accurate enough set of calipers you can grab expected ID and OD of the intended O-ring to replace it.

Nose grease works fine on those ferrules. It would not be a bad idea to replace the o-ring. Just take it off and bring it to the hardware store, then have the plumbing guy find you the ring that fits. Before you do that though, rub plenty of candle wax on the male ferrule and the o-ring and try to seat it. I have had those ferrules seat after waxing, when they would not fit without.

Sometimes it works and sometimes it doesn't - don't give up on them.

The ones I am familiar with were anodized aluminum, so probably expansion and contraction affected fit regardless of manufacturing tolerance, which is probably more difficult to maintain anyhow with the surface anodize. The O-ring ("size-matic") compensated. Try a little experiment. Ice the ferrules for a few minutes, wipe them dry quickly, and try it. Let them return to the ambient temperature they were at when you had the easy fit. Try the fit at intervals as the ferrules return to that temperature.