You probably need a PC tip conditioner.
https://paramountcords.com/product/pcc-pc-tip-conditioner/
I'd check to see if Paramount is actually still in business - I have some doubts.
And I paid a lot less than this for mine.
I'm not sure that's actually available, unless they are clearing out the last few bits left in stock--I e-mailed Paramount back in December of 2020, and got a message back saying "
Thank you for your inquiry. However, Paramount's operations are permanently closed."
A PC tip conditioner is nice to have in general--but it's *not* what you need for a Rolleiflex. Rollei had a special proprietary male PC tip that locked into the socket on the camera--that's why a regular PC sync cord will fall out, the tip conditioner really won't help with that. Paramount used to make flash cords with the Rolleiflex locking tip--and in fact, I have one to use with my 2.8F connected to my Speedotron power pack that I ordered from them. If you look
here (or at the photo below, which is larger), you can see what the Rollei locking tip looked like. Notice, it's a lot larger in diameter than a regular PC tip and right near the very end it had a ring around it--that's what locked it into the socket, and why the socket has the spring-loaded lever surrounding it so that you can insert and remove the tip. (It's a shame Paramount has closed down; they made excellent-quality cords, and you could custom order them with whatever tips you wanted at either end. I ordered one with the Rollei locking tip at one end and a standard phono plug at the other, so I could easily directly connect my Rolleiflex to my Speedotron gear. I still use it.)
There are adapters you can occasionally find that will convert a regular PC tip to a Rollei locking tip.
There is one currently available on eBay, which appears to convert the screw-collar locking type PC tip to a Rollei locking tip. (By the way, I have bought from that seller, who is located in Israel. I can vouch for them!) My adapter (I also have one I used before I ordered a custom Paramount cord) is a lot simpler, it looks like this, and will work with pretty much any standard PC tip:
On the left, there is a regular PC female socket you plug your standard cord into, then you plug the Rollei locking male tip into the socket on the camera. It's not as nice as a proper cord, as most PC tips (the plastic bit surrounding the electronics) are too big to slide fully into the receptacle on the left, and so they also tend to fall out in use--although not as much as trying to use a standard PC cord in a Rolleiflex socket, so they work well enough. That one on eBay looks kind of nice, if your PC cord has the locking screw collar. If not, I'm not sure it will work. If you've got an old camera shop around your way, the adapters like mine should be relatively easy to find.
Edit: there are also Chinese eBay sellers offering sync cords with a variety of tips for plugging into flash gear, and a Rollei locking tip at the other end:
here is one. They seem the cheapest, but there are several other sellers also offering Rolleiflex-style sync cords, if you search.