yeah they pretty much summed it up ^
means you just need to trim a little off the ends and beginnings of the clips you are trying to fade/dissolve. for the fade to work, video is still being played while opacity is changed in the transition. If the video stops and there is no more footage on the end of the clip, there isn't video to dissolve into the other one, it only has enough video to work with on the front end.
Best way to fix it is by just trimming the ends on each clip back a bit, then re-connecting them and re-adding the cross dissolve or whatever transition