but everything else is fine. how do i fix this?
Try taking a small amount of thin cardboard or paper and slipping it up inside the bore at the bottom of the chanter. It should be about an inch long, by about 3/4 of an inch wide curled enough to stay inside the bore.
Experiment with the location and the length of the paper. Don’t cover any of the bottom tone holes.
If you don’t like the results, you can easily remove the paper without changing the chanter or reed in any way.
It could also be that the lips on the reed are too close together.
Has the bottom D ever worked properly?
If not then it sounds like the throat is too large - try dangling a length of thread down into the throat of the chanter, reasonably thick waxed thread works well.