Some things just don't bind up well independently. I suspect you just aren''t snugging the ropes down tight enough. Practice helps and to that end it pays to experiment with alternative bindings.

I suggest five to six loops around the body, three below the breasts, three above, criss-crossing behind the back. Tie that tight and with a snug knot, (in back.) If the rope is long enough, or with a second rope, come back up front and tie the upper tier to the lower tier, between the breasts, with several wraps of the rope, tightening everything up by bringing the two tiers together, touching if possible.

See how that works for you.... If it does, then you can try making it even tighter by snugging the tiers up to each side of the breasts.

Then tie the arms behind her with a dragonfly arm binding... that will make the chest bindings even tighter.

Pay close attention, breathing will become labored as the chest becomes more and more constricted.