Distributing audio over slides

I recorded a narrative for an entire slideshow of approx. 75 slides, then began to distribute the narrative over the project's slides using the Slides Audio box. This worked well for the first 19 slides. In trying to place marker for slide 20:

1. The Start next slide at the curosr position button no longer works and
2. Dragging the slide divider for slide 20 results in the message "You have reached the minimum diplay time for slide 19"

Any assistance would be greatly appreciated.

Thank you.