Walt,
A few possible ideas here.
Is it heavy enough? You can also test this by gently pressing down on the center of the engine while its slipping with a finger. If it pulls away, might be the problem. If it keeps slipping, you should probably put a new traction tire on it.
Next, try flipping the engine and running the other way. Sometimes the traction tires do terrible on the inside of a curve (if its a single tire on one side).
Something which can happen quite easily, is lube spinoff onto the track, thereby getting on the tires. I'd probably clean the tracks with QD (Quick Disconnect) cleaner or Lectra-Motive electric parts cleaner on a rag or some other plastic safe de-greaser.
Then there is the horrible dry-rot tire.
