Problem with strapless wedding dress! Advice badly needed!

Just looking for some feedback- I got a strapless dress and I noticed that the “bodice part” seems to sit a bit too low.

It’s hard to explain, it’s like the bodice part slips down an inch too low for the dress and it causes a crease at the bottom of the bodice? The dress itself looks well and it’s not too small/tight one me. No matter what I do the crease at the bottom of the boning seems to appear.

I brought it to alterations who told me it was my stomach, which really upset me, and that all I could do was get Spanx or similar. I did and they made no difference...

I know it’s not a big deal, but it’s made me feel a bit self conscious. Does anyone know why this is happening or (even better!) how to fix it??

Thank you!!