I do not own a lion chief plus system, so I'm not certain how the LC+ implements an antenna inside a cast metal shell. But I can tell you that of the lion chief systems I do own, they use an antenna that is etched onto the circuit board in the engine, and there does not appear to be any option for an external antenna. My guess would be that your transmitter or receiver boards may have an issue. Have you tried using a universal remote? Does it perform the same/better? The receiver board in the engine is not very expensive, but availability could be an issue. FWIW, my LC (only) engines function well at about 25ft - that's as far as I have tested them.
The LC systems I have are probably working as they are supposed to. I need more distance for my layout and my club layout. So far I have tried adding an external antenna to a receiver to get it out of the cast shell, but I don't think it made any difference.