[laminar]

The only thing that comes to mind is that the change in air volume in your lungs and ears/sinuses is much greater in the 0-15m range than it is after that. From zero to 10m, the air inside you is compressed to roughly half the original volume at the surface. Using Boyle's Law (P1 V1 = P2 V2) you can see that the change in lung volume decreases less and less as you go deeper.

Perhaps you are waiting a bit too long to equalize after each successful equalization? But then when you get deeper, the pressure doesn't build up quite as fast, giving you more time to get in an equalization.

It is possible to overpressurize your ears so they won't equalize at all. That used to happen to me.

How do you equalize?

Pete