Thanks for checking out my tutorials, Tommy!
Now zoom into your timeline at the spot where you're trying to add your transition. There may be an extra clip (just a frame or two) between the two clips you're trying to add a transition to.
If that's not it, then please describe exactly what you're seeing when you try to add a transition.
Please include in your reply to SG whether the two clips involved in the specific transition plan are video/video, video/still, still/still. And, what alignment are you trying to get for your transition, Left Clip, Between Clips, or Right Clip?
When checking the Clips, per Steve's suggestions, also look for any gaps, between them, in the problem areas.
I just zoomed in - stretching out the timeline. Now I had no problem inserting the transitions when I had more space to work with. Easy solutions like that are satisfying.
Great news, and thank you for posting of your success.
Good luck, and happy editing,