There are times when I make changes to individual clips within the timeline, but I am then not able to put a clip to clip transitiion onto my new cuts. Why does this happen and how do get around this problem?
Make sure that you have extra frames available at the beginning and end of each of your clips. If your in point is at the very beginning of the clip and your out point at the very end there is no room to add a transition.
Don't I use the viewer to establish my in and out points, then drag the selection into the timeline, butt it up against what ever clip comes before and after, perhaps make some detailed frame adjustments, then insert an effect if desired such as a cross fade? The problem is happening when I un-do an effect or make a cut, replace or adjust one of the clips, then putting an effect crossing into both clips. It seems to work if I put the effect only on the ends of each individual clip (which doesn't yeild the same result), but sometimes, and I don't know why it happens inconsistently, the effect will not place over both clips after I've made adjustments in the time line. It's like I have to start from scratch to get the effect to cross over into both clips. I'm confused as to why this happens sometimes and sometimes not.
You're right about using the viewer to trim your clips & bring them into the timeline, but the the effect can't cover both clips is because either one or both of the two clips don't have the media to cover the dissolve.
You have to think of it as if you're editing film--if you're going to dissolve between two strips, you end on the frame you want, then add 10-15 frames, overlap it with the first 10-15 frames of the next clip & you have your transition. You can't just put them back to back, because you need the overlap for the transition to happen.
If you're doing a ripple edit to a cip (changing the in or out point), then you need to make sure that there's no gap between the two clips that you want to transition between.
Otherwise, as others have already said, make sure you have enough unused frames (at the end of the first clip and at the beginning of the second clip) for the transition to use.