This is a classic mistake regarding rounding and casting. Rounding a position in space or casting a fraction to an integer, results in the integer discarding any fractional information. What happens is that your delta is too small for the position to significantly update to a new integer position. So, when you round it, it will round to the nearest integer. In this case back to its original position. Try to avoid rounding when it comes to position. You could also increase the gravity to see when it begins to move.