I did this game below and I strangely can't shoot the opposite direction after pressing UP or Down keys.
I've sent the project file so you can inspect the game. Thx.
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As you have only sent the file to scirra, you might have to wait a while until you get help. If you post the capx here, the community would also be able to help.
From what I saw, it seems to me that you have just invalid angles set for shooting.
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Thx for the help. I added 180 degree to the bullet and it's all fine. Some updates.. check it out.