Ghost trigger

I'm having problems with a trigger on a slide layer in Storyline 2.

The intention is that when someone moves the mouse pointer over a piece of text, a layer is shown with a feedback comment. There are 5 such pieces of text, each related to a different layer. Four of them work! [Show layer x when the user hovers the mouse over]

When the pointer moves over the 5th piece of text the relevant layer is not shown, and  the focus jumps to the underlying base slide.

After carefully checking the trigger details to ensure I hadn't inadvertently set the wrong trigger, I have:

a. deleted both the trigger and the relevant piece of text, rewritten the text and then set another trigger.

b. deleted  the feedback layer and produced another one

c. moved the piece of text to another part of the layer, in case the position was relevant.

d. set the trigger to show a different layer - that worked! But I can't get it to show the layer I want, even after having deleted the original later and replaced it.! I even renamed the layer, in desperation, but that didn't work either.

Rather ironically I mean to have a button that will enable the user to return to the base slide when they're finished, but that hasn't been set yet.

I've run out of ideas so would welcome any suggestions.