Buttons not showing up when supposed to on time line

Each slide has a button that is supposed to show up near the end of the slide. So we created the button and placed it on the timeline in the appropriate spot for each slide, so it shows up 2 seconds before the end of the slide. Strangely though, when we play the courses, only some of the buttons appear when they are supposed to. Most do not show up at all even though they are clearly there in the time line. Any ideas on why this might be?

Thanks in advance,

Arthur