Check your publish settings for png and make sure you have 24-bit with alpha selected and smooth on. That should fix the problem.
It didnt work unfortunately but thanks for trying to help.
It's because the png images are too small INSIDE the graphic symbol. You need to scale up every frame of the symbol to make them bigger in the spritesheet.
Double click the symbol containing the PNGs then select all frames using the Edit Multiple Frames button above the timeline (or press Alt+Shift+E on your keyboard), now scale up the drawings a little with the free transform tool (Q) and try generating the spritesheet again.
Thank You it worked!