On descent, the air in the middle ears is reduced in volume by the increasing surrounding (ambient) pressure. Some divers can equalize their middle ears easily, but on occasion every diver has had some trouble. Others always have difficulty.
The culprit is the inability of the Eustachian tube to open. The tube extends from the middle ear to the back of the throat, above the palate. It is lined by the same kind of lining as the nose and sinuses and is subject to any inflammation that can occur in those areas. If there is enough swelling or mechanical obstruction, then equalizing can be difficult, if not impossible.
To avoid difficulty, you need to control all acute nasal and sinus problems. If cleared by your physician, using decongestant medications may be helpful. Equalize early and often. Use proper techniques.
Don't let your buddies rush you. If you do there may be permanent damage.