Originally Posted by FBinNY

I can't speak for others but am curious if you mean that the chain slips immediately after freewheeling, because how does a non moving chain slip?.

Slippage after freehweeling,or failure of the hub to engage solidly could be sticky pawls, and is usually solved by flushing the freehub, to clean and remove dried grease, then re-oiling with a heavy, sticky oil or very light grease.