You are correct, the engine should work on it's own. The middle cars do not have pickup rollers, only the tail car and that is not required for testing or operation - caveat though, you do have limited pickup rollers with only the engine.
It sounds like you either have dirty wheels or it could be the pickup rollers. They can get black oxide or rust on the rivet shaft of the roller and not make good contact. This is a used engine. The typical fix is using a very light or "conductive rated thin oil and then using a Dremel or other tool to spin the roller with some pressure.
Also, a factor in this testing, the engine by itself only has one pickup roller so a switch or other track feature could easily result in a loss of power to the engine.
I don't think you need to send back, I think you need to clean the rollers and wheel, lubricate with very light oil, and test using the engine and tailcar as a set to get at least 2 pickup rollers on the track.
