When a less-than-amply-endowed (or endowed at all) girl lies on her back and her boobs retreat to somewhere unseen betwixt the armpits and the ribs.
Major cause: missionary position.
Short term solution: avoid any positions where gravity works against you and instead use it to your advantage in doggy, woman on top, or other inventive style.
Long term solution: get yourself a pair of Pammy implants and gasp in amazement at how your boobs never move an inch out of place no matter what challenges you throw at them. Weep in dismay when you realize that you now resemble a bottom of the range porn star and can no longer sleep on your tummy.