Inconsistent hover / arrangement problem

I have created a small menu at the bottom right of the attached file, in the slide template. Each of the four icons in it has a hint on hover. For some of these (for example, the clock icon), the hover caption appears as it should - arranged to the 'front' of the other icons. For others (for example the gannt chart icon), the hover caption appears behind the other icons.

This is despite whatever I seem to do in editing the hover state. I may be missing something obvious but I would be grateful for advice!

(Also to note, the light boxes referenced by triggers on the template, are not present, but this makes no difference to the problem)