How about You just try to replicate the problem in a completely new .cap.
I think it's a good habit to, before implementing something new to a bigger project, make the addition work in an empty project file, so You can always be sure, there is no other code interfering.
That way, if it works, it should work when You implement it in the bigger project, and if it doesn't, then You know some other code might influence it.
Also, in case You still don't understand why it doesn't work, when You only do this one thing in a new .cap (which also gives You a more clear overview of everything), when You ask for help on the forums, You can just post that new .cap only showcasing the problem.
And along the same lines, before implenting, meaning, when You test Your idea in a new .cap first, You don't always need to go full on and add sound and graphics. Unless You wanted to test something related to that. You know, don't waste Your time with unecessary things.
For example for Your problem now, You could just quickly double click the empty layout, add a new Sprite Object, in the Sprite Editor just fill the empty rectangle with some color with the bucket, add a colored dot on one side (left or right), to see which direction it is facing, save. Add another new Sprite, resize it to make it a little smaller, and fill again, save. This will be Your projectile.
Now You have one big square with a dot (Your Player), and one small square (Your projectile).
Now add Your MouseKeyboard Object and add the least necessary Events to do what You want to do: Turn left, turn right, shoot projectile according to the direction Your Player is facing.
That'd just be about 3 Events, and everything together should just take You about 5 minutes to create, and You'd know if it works or not.
Usually, You don't even have to rewrite the Events, when You want to add them to Your bigger project. For this to work, You just need to rename all the used objects in Your testcap to the same names they have in Your game. And if You added variables that the objects don't have in Your original bigger project, You just need to add the new variables to the related objects in Your game.
Then You just select all the Events You want to copy from Your testcap, right-click on any of them and click 'copy'. Then You go to Your games' Event List and right-click paste them.