Thanks for all the suggestions, folks!
I failed to mention that I bought the press used, and that the 11,000 rounds was only what I made with it, absolutely no idea how old the press is, or how many rounds were made with it before I bought it. My guess is the press is around 15 years old, with many thousands of rounds through it. Since I've had it, it has been disassembled, cleaned and lubricated regularly.
Today I disassembled the primer assembly again, and I noticed that the two Allen bolts holding the primer assembly in place were pretty tight (don't know how that happened!)
. I backed them off a little (while still keeping them snug), and that seemed to free up the slide a little. I compared the tension of the return spring on the press with the spare spring from the spare parts kit (which I was smart enough to order a while ago!), so I replaced the return spring with the spare spring. I haven't had a chance to try this yet, but I'm hoping that the mounting screws and the spring will help solve the problem.
I will definitely take a look at where the actuating rod sits and adjust it if necessary, thanks for the advice.