My handwriting recognition model performs well in desktop browser, but terribly in mobile browser

I have started to develop a math learning app that converts the learners hand written digits as they write. It works really well on a desktop/laptop browser (>95% accuracy) , but performs very poorly (<50% accuracy) in a mobile browser whether it is chrome or safari.

I have checked how their strokes are being translated to the image sent to the model, and this seems fine. I’m really not sure what to check or where to look.

you can try it out here. click on try the demo:

What could be causing this? Are there any solutions I could try?