Tried the 'tipping trick' with no joy, also tried tapping the tube in different areas. Still the same result no matter what I do, which seems strange. I would have thought it would move at least a little, but it might just be jammed in there really well.
Here's pics, as promised.
First one is blank raster, brightness cranked up to see the splotches. Then I turned on the guns one at a time: first blue, then green, then red. Red is the most telling of course, you can clearly see the edge of the shadow mask off to the left.
5th photo is actually an animated GIF, so you can see the progression.