Numeric variable trigger is not affecting state of Next button

Thanks for sharing the screenshot.

Without seeing the full slide, the first thing I'd suggest is to add your reference variable for %Items_Castaway% to your slide to see if it's counting correctly.

But the thing that stands out the most is the trigger order and how you're counting the selected items. Storyline's Selected state should work because it automatically toggles between Normal and Selected. Here's what I'd try:

I attached a quick .story file you can look at to see if it helps.