Let's be clear: Sinking is not "swimming down"; it is letting your body descend naturally with the correct posture and weighting. If you have to claw and kick your way down, that means your weighting or posture is wrong.
Why can't you get down? Two causes are most common: first, not enough weight, so your wetsuit keeps you afloat; second, equalization isn't working—as soon as pressure builds, your ears hurt and your body instinctively wants to go up. The second is more dangerous; forcing it may injure your eardrum.
- Start with a duck dive. Enter the water with your upper body vertical, lift one leg to create leverage, and let your body weight take you down. This is far less tiring than pulling with your arms.
- Tighten your posture before you kick. Keep your arms close to your body and your legs together; the tighter your streamline, the less drag. If you kick loosely, all your effort just goes into the water.
- Equalize first; if it hurts, ascend. Descend along the line. If you cannot equalize, ascend immediately. Never force equalization just to gain depth.
In short: with the right weighting, working equalization, and a tight posture, the water will take you down on its own. When practicing these movements, an instructor or a buddy with rescue capability must be present. Practicing descent alone is one of the most accident-prone parts.