Cup gaping but bra too small?? Help

So my measurements are 30” under bust 35” over bust making me a 30DD sister sizes 32D 34C 36B. Although I am a DD my boobs are rather small I have no over boob and very little under boob. Some push bras work at pushing them up or together but I’ve been having a really hard time finding the right strapless bra. I recently bought chicken cutlets(bra inserts) to stick into my bra to push up. I have an old 34B bra strapless bra which obviously doesn’t fit correctly but fits ok, I stick the cutlets and theres as huge gape from my boob and the bra it looks weird in shirts. The bra is obviously tight cup with the cutlets in so why is the so much gaping happening?