Thing with hermits is it's not about the quantity of shells 🐚 in the tank it's about what one they want,
They are mischievous but very funny to watch ,
You say you have spare 🐚 in the tank,might pay to get some that are a bit bigger ,not to big obviously but they do grow so need bigger shells 🐚.
Try that ,sooner the better or they might start attacking you're snails for there 🐚,try to pick a few weird shaped ones aswell they seem to like them,
Hope this helps,good luck,