Hi The Bockies,
You are dealing with canvas, it changes its shape, shrinks, expands all the time. Also, the ground can make a difference. Slight lean on way or the other and the tent is out of shape. Nature of the beast.
We can set ours up in the morning and by mid afternoon is all askew, late evening, pretty close to good again. Sometimes can stay in shape for days on end. depends on the environment.
Get hung up on having a perfect 'tent' and you will only get annoyed. Constant change.
If the sag over the bed is annoying, you could get an extra pole or two to keep the shape in place. We do and it works for us.