The amount of lift made has a lot to do with the angle of attack, the angle between the relative wind and the mean chord of the wing. While the space shuttle is in gliding flight, it flew with a very nose high attitude in a reasonably steep descent, thus the angle of attack. Under rocket power on ascent, the relative wind would be coming pretty much nose on, so a very low angle of attack, thus very little lift.

If the angle of attack goes negative, the wing will lift in the other direction, which is how planes can fly upside down.