It's a standard medical procedure called an air lock. As long as you are in the muscle(you must asperate) the 1/4cc of air will do no harm and will force all the fluid into the muscle dispersing it. Also inject slowly. But the main culprit is probable scar tissue. How long have you been hitting this site. After a while the injections just refuse to absorb and the are becomes nearly undispensable for injections, so it has a noce hardened path to flow back up through as the injection wound doesn't close. Rotate sites and give the glute a rest. Shoot in the quads, delts, and bis for more sites thus limmiting the impact of frequent injection in one area.