Hi,
yes, because it doesn't have time to shwo you the difference, since it's only different between the time you get the current position and the time you overwrite the previous position with the current, so it's like a matter of less then a milliseconds, while the rest of the time it's equal.
you need to store the real position into the "current position" first, then perform a substraction with the "previous position" and then store the "current position" in "previous position".
yes?
bye,
Jean