Assuming you have the original levers on it, are the levers metal or plastic? I have seen both on Troubadour III harps.

Sounds like the lever is not making firm contact with the string. The lever or the string post (or both) may need adjusting. If there’s any chance you could get a harp technician to check it out, that would be your best bet. A good regulation would probably fix the problem, especially if you’re not comfortable doing adjustments yourself. Meantime, you can try putting a little bandage tape or a tiny piece of gaffers tape or duct tape on the end of the lever to build it up only where it touches the string.

My experience with those metal levers (the larger, thick, gold-colored ones, not the small brass ones on Troubadours I and II) is that they were a composite metal and brittle. Several broke so that there was not enough to grasp to move the lever. Replacements were not available so we finally had all the levers replaced with Loveland levers, the most practical and affordable fix at the time.

You should consider getting Dave Kolacny’s book “Trouble-Shooting Your Lever Harp,” which addresses most problems with many brands of lever harp. It’s available from Vanderbilt Music, among others.