A convertible bra that stays? Plunge/demi cup with gaps?

I'm barely 5 feet tall, 95 lbs petite woman. When I was sized at Aerie and VS, they told me I am 32A. Victoria secret's PINK everyday bra rather fits me pretty well. I like to wear strapless/ halter dress or tops, and I wanted to get a strapless bra. I got 32A Convertible from Aerie, and it slipped an hour after wearing. Same result from Gilly hicks. So I gave up on buying more convertible bras, and started to looking into demi/plunge bras, so at least I can wear deep cut shirts without showing my bras all the time. My boobs are fuller on the bottom and with all of the demi/plunge bras I got, whenever I moved around, there were gaps on the top of the bra.

Is this normal with wearing plunge/demi bras? Also, can I ever find a strapless bra that does not slip?