Master Slide layer gets hidden when go to new slide, help!

You need a trigger on that next slide to open the layer. When you create a new slide, it is not just a view of the Master. It is its own separate slide, that has on it everything that is on the Master. Therefore when you show a layer, it isn't showing on the Master, it is showing on the current slide. When you move to a different slide, it isn't showing on that slide. You need to pass a post-it note to the second slide telling it that the layer should show, and a trigger to show the layer if the note says it should be open. In SL, post-it notes are called variables. You only need one, a T/F. When you want to show the layer, use the same action to set the variable to True, and when you close the layer, use the same action to set the variable to False. On each slide, create a trigger to show the layer when the slide timeline starts, IF the variable is True.

You might experiment with setting the variable on the layer in the master: True when the timeline starts on the layer, and False when it closes.