Slider Interaction for syringes

As part of a module to help nursing students practice medical math calculations, I created an interactive that includes a working syringe. I used the slider tool (then formatted the Thumb Fill with a picture and adjusted the track length - kudos to this post for helping me figure this out). Everyone loves this interactive! 

However, over the past year, we have had about 100 students complete the interaction and around 10% of them have problems which seem to be random browser issues, old flash versions, and possibly mobile device access. When learners switch to a new computer, it always works. But this is not an ideal solution, so we’re looking to update or change the interactive. I wanted to get some advice first.

• Am I doing something crazy here? (entirely possible) Did I miss something? Should I switch gears?
• Would updating to Storyline 3 help? These were created in Storyline 2.
• I considered creating a test file and getting learners to help identify more specifics with devices and browsers, but I'm not sure it's worth the effort. I also considered hosting a test file off of our Blackboard LMS to remove that as a factor. 

These are published as Web with HTML5 output (and no Articulate mobile player). Grateful for your feedback or advice!