Do you have handles on those clips? Something a lot of new users struggle with is the concept of handles.
Transitions work by using a "bleed-in" on the second clip while "bleeding out" the first clip. But there has to be clip there to use. So if you have simply butted two un-trimmed clips against each other, there are no handles for the transition to use.
If you trim back a second or so from the end of the first clip, then trim in a second or so on the second clip ... you now have "handle". Which means that your transitions can work properly.
Look at my example ... see the little triangles in the upper corner at the ends of some clips? That means the clip use on the sequence goes to the end of the full clip ... or starts at the beginning of it.
See that middle clip? It has no triangles ...and the beginning of the third clip has no triangles.
So a transition would work between the second and third clips (which both have available clip for the extra frames needed) but would not work between the first two clips as the fist clip does not have any extra frames.