Doing mitre cuts on plastic window sils

Have you checked your saw for a perfect 90 from the blade to the fence?

Problems with miter cuts are usually related to improperly adjusted saw (see oh'mike comment), or improper measurement of the required miter angles. Not all corners are 90 degrees, you need to measure very carefully first, or make some test cuts and adjust.

Is this the stool or the sill your cutting? The stool in inside the sill is outside.
Try to figure why you would be using plastic on an inside stool and not wood.

maybe using azec so htey dont have to deal with condensation ruining the paint finish

It also depends on how the trim sits on the window jamb and the wall. You may have to fit the miter first, may have to shim behind the meeting miters, then caulk the space the best you can.
I assume you checked for square saw blade, same size trim, jamb corners that meet flat, wall board with too much compound, plaster, etc.
Homedepot trim can have slight differences in size from one piece to another, even within one piece.