I do not know about the most recent two decades...but the Horners 1960-70-80 ...all that I have,,, have used plastic, it is easily identifiable, it is a thin white-strap.

Swishing and twirling in water will work to "unlock" a stuck valve....however, if you examine the metal reeds, you may discover "corosive" bits of mineral/rust type deposites....

IF, the pitch has changed... or a hole is off-key, it is probably this corrosive-crap on the metal....not on the valve-flap. You can scrape it...and you can use the "vinegar cure" and also scrape it.

Old Harps, are ol' friends...A half dozen lie on my shelf...they sit awaiting a day of ressurection and redemption. They have served me well and are worthy of a rest....However, when suck comes to blow...they will let me down at some point...early.

Fortunately, they had many clones...and are much cheaper than mandolins.

Sincerely,
Gargoyle