Variables: Do not use periods
I'm sharing this info in hopes to save someone hours of troubleshooting.
Normally Storyline will not let you use a period when naming a variable. However, I copied/pasted the name of a text-entry field from the timeline to the variable name. It saved just fine.
When previewing, it caused the slide before it to not advance. We assumed there was something wrong with that slide. After hours of troubleshooting, we played each individual slide and realized the one with the text-entry variable wouldn't play in preview. I renamed the variable without the period and it works fine.