Zoom into the timeline at one of these places.
Often, when arranging or re-arranging photos on a timeline, if you drag one clip too close to another, you actually split it and a small sliver of a clip is between the new clip and the next. When you try to apply a transition, the program can't apply one to such a small clip.
You may also see these accidental "phantom clips" between your main clips if you temporarily switch over to sceneline mode.
Also per the FAQs to the right of this forum and my books, if you don't ensure that your photos are no larger than 1000x750 pixels, you will overload the program and get some pretty strange behavior. Are your photos larger than 1000x750 pixels in size?
Thank you Steve for your prompt response. I have found some of these
slivers that you mentioned and deleted them. Also, previously, I have been
inserting transitions in the sceneline. A friend recommended using the
timeline which gives a better indication of what's going on.
As I say in my books, Timeline mode is, 99% of the time, the best mode for editing. However, occasionally there are advantages to switching over to Sceneline mode once in a while.