A few thoughts.
I have wasted money on strapless bras several times.
They just don't seem to fit or give any real support, they slip down and I believe that most GGs have similar problems. Also, with these bras, the straps are not sewn in, but attach to the cups and band with flimsy plastic hooks. The hooks can slip out or snap. I had a bra strap fly out from my top with a loud ping when the hook broke!
I find that with any new bra, the straps need to adjusted to their limit to start with, but quickly stretch. Washing the bra prior to wear should help.
Also, bra straps need to be surprisingly tight to be effective. If you can stretch the strap to the bottom of your ear, it's too loose. Your bra straps DO provide a lot of support and shaping, and if they didn't, then everybody would be strapless, or multi-way, which they are not!
So it just might be that if you are new to wearing a bra on a daily basis, then you need to get used to tighter straps than you thought.
I am currently breaking in two new bras which were suffocatingly tight, but now need regular strap tightening to avoid the horror of the dropped strap with a sleeveless top.