Button does not appear at time set in timeline
I have a series of slides with .wav files containing lecture voice overs and a 'Next' button on each slide.
I did not create this presentation but have been asked to amend it so that the user must listen to the entire lecture on each slide before the 'Next' button appears.
The button should appear 2 seconds before the end of each .wav file. This works on most of my slides, but there are two on which the button never appears and I can't work out why.
Any advice would be greatly appreciated.

