You jump off the boat, turn to your buddy and give the down signal then start descending. No more than 2 feet down you feel it, the pressure in your ears. No problem you think, just pinch and blow and everything will be right as rain. But that glorious squeak doesn't happen and your buddy is already at 30 feet.
Equalizing is something every diver does and what many divers have difficulty with when they are beginners as well as advanced. You may have equalized without even realizing it on a plane as it prepares for landing. I have had days when I had a hard time equalizing and there a a variety of reasons that can cause that. The first place to start is proper technique.