.... it is not the code or the hardware, but either the design ot the implementation.

If you wave various items in front of some sensor at a varying distance by a motion that is not rigourously controlled you will, on occasion miss items and/or doublecount items.

The solution is a rigourosly controlled motion of identical items or a rethink of the design.

I do not know the exact conditions, the OP never said, but if debounce is possible that may be a solution.

Erik