Video transitions not being accepted on adjacent timelines

This is my first time trying the technique of working with multiple timelines, one for each segment of an interview. in compiling my main edit, adding each timeline/segment sequentially, for some reason I am not able to apply a video transition between each segment. I click and drag it over the edit spot where I want to insert it, but it doesn't get accepted. I would appreciate some help in understanding why that is, and what the solution would be.

Many thanks!

You need overlapping frames for the transition to work within eg for a 1 second transition you would need 1 second of overlapping frames.

This is a rough video I made some time ago (I really must update it) about transitions. It may help.