When changing the length of a fade transition between clips, the actual duration does not change.

I am trying to make a short clip with two different videos and a fade transition between the two. I have placed a fade transition between the two clips (which are plenty long enough... 30+ seconds on one, about 8 seconds on the other). On the mobile version, I cannot change the duration of the fade between the two. On the desktop version, I can extend the duration of that fade transition as long as I like, but in actuality, the slowness of the fade changes not a jot. It seems stuck at the original setting, no matter what Premier Rush represents. When rendered, I get what I see (a very short abrupt fade), not a long fade like the desktop app represents. How can I get either version of the app to actually give me the fade transition duration that I want?