Welcome to the forum.
There can be several reasons that a Cross-Dissolve (or some other Transitions) might not work.
One of the most common is that the user just makes a Cut, but does not alter the footage, and when they place the Cross-Dissolve over that Cut, there is not change, as the Frames have not changed.
The next is when one adds a Cross-Dissolve between Clips, that do not have adequate Handles, extra Frames, that are necessary for the Transition to function properly. This ARTICLE goes into more detail on Handles.
Do either of those situations have anything to do with what you are experiencing?
If not, can you post a screen-cap, showing the Clips, to which you are having issues with the Cross-Dissolve Transitions, and tell us exactly what you have done, prior to the addition of the Transition?
Thank you for reply - have now managed to insert transitions.
Great news, and thanks for reporting your success!
I suppose that you can mark your thread Answered, so that others will know that you found a solution.