Coving - what am I doing wrong

Afternoon folks.

Yet another coving post.

Attempting a DIY coving install however there’s such a big gap between the corners and I can’t figure out what I’m doing wrong.

I’m using a coving mitre box… could it be that the wall isn’t 45? Can’t for the life of me see what I’m doing wrong.

When butting the two pieces together off the wall they seem to line up much better (not perfect!).

TIA

Afternoon folks.

Yet another coving post.

Attempting a DIY coving install however there’s such a big gap between the corners and I can’t figure out what I’m doing wrong.

I’m using a coving mitre box… could it be that the wall isn’t 45? Can’t for the life of me see what I’m doing wrong.

When butting the two pieces together off the wall they seem to line up much better (not perfect!).

TIA