If your hotfix rhinestones are falling off, there could be a number reasons.
1) They may not have been adhered correctly, for example not enough heat was used to melt the glue fully, or there may have been some dust or fluff on the material which prevented the glue from properly bonding.
2) You may have accidentally knocked or scratched the rhinestones too hard against another surface.
3) If it is a garment, it may have been overused or over washed – garments with hotfix rhinestones attached can withstand a cool wash, but fabric conditioner will loosen the stones so avoid using it if possible.