Martin is correct. Mellotrons are not supposed to have problems. There are not supposed to be "wobblies" or stuck notes. A proper Mellotron should pull the tapes through and deliver the sounds as intended. The character of a Mellotron comes from its electronics, not its mechanicals.
But Frank, you do have to admit that there is SOMETHING about feeling those tapes moving under your fingers that lends something to the way you approach playing it. For some reason you don't feel the moving parts of a piano or harpsichord the same way. A tracker organ (i.e.-one where the keys articulate wires opening the pipes to the air chest) has a bit of that nice feel, but it the mellotron that wins in this regard. The Chamberlain would, too, if the keyboard didn't feel so much like a toy (mind you I've only played a couple of M1s).
Rick