If A is the first clip and your intent is to circle effect it to the second clip, B, click the X transition icon that's in the A clip.
Can't say why transitions were lost, but suggest that you save your project frequently, with FILE|SAVE AS (not FILE|SAVE).
Increment a number for it each time, so as to not overwrite earlier versions. Project 1, project 2, project 3, etc.
To insert multiple transitions, first select (highlight) the appropriate clips. Then click TRANSITION/S in the menu or the toolbar (not on an X in the timeline clip) and click on the desired transition.
You should be prompted to APPLY TO ALL or APPLY TO ONE. All will be the same duration - the length shown in Transition Properties box at the left-bottom of the transition window..