amz
Hello & Welcome to the Forum!
Sorry to hear about your slippage problem.
Adding weight to your locomotive is one solution, adding traction tires is another.
Are the cars you are pulling heavyweights? are the axles & wheels on them well lubricated?
that is the problem l discover when l run into slippage, trying to pull too heavy a load for the locomotive, or no luberication for the axles & wheels making a diffucult pull.
There are a variety of reasons, from out of gauge track or wheel sets to too step an incline or slippery track that might cause your wheels to spin.
you just have to keep trying until you discover a solution to your problem.

happy rails...
dave