When handling a wifi connection, the CPU needs to do more work than when handling an Ethernet connection. So in situations where large amounts of CPU resources are also needed to handle the stream (high bitrates, small packet sizes in the container, etc), there may be videos whose playback consumes too much CPU resources on wifi but can still be played on a wired connection. This is independent of signal strength or network bandwidth. Unfortunately there's not much that can be done other than choosing a less CPU-intensive encoding.
--Mark
