To be simplistic
- wave passes underneath you - you are too far back (away from the beach). Remember that you catch a wave through gravity (going down the face). So you need to be in a position to slide your board down the face (use gravity). If you are too far back, then the slope of the wave isnt steep enough (assuming the wave breaks a few metres further in. If it breaks 20m further in, then you just picked the wrong wave)
- you nose dive. 95% of the time, because you arent paddling fast enough to be in the right position when the wave hits you. It lifts your tail and your nose dives (so its not your nose diving, its your tail lifting)
As waikikichan often says, catching a wave is timing, positioning and paddling. You can catch a wave just with the first two (eg no paddle takeoff), but most people need paddling to fix up their timing and positioning (and also waves break in different places so you rarely will just happen to be in the exact right place)
So that is the cause. Wrong position, not paddling fast enough.
What is the solution - well, depends on why you are in the wrong position. But first step is to make sure you are getting the basics right:
- position on board, make sure its right -
https://barefootsurftravel.com/livemore ... -surfboard- position in line up - this can be hard, but if the wave is passing beneath you, then you need to be closer to the beach when the wave arrives. Either paddle harder, paddle earlier or start closer
- paddling - make sure your paddling is efficient (see link and waikikichan's post above). Keep paddling until you have caught your wave. Remember you need to be going down the slope, so you may need to still be paddling when your board starts tilting to go down the slope. Thats fine/what is needed. Dont stop now or you will miss the wave or nose dive. Maybe you need to start paddling earlier to get up to speed; on steep shallow waves you might need 4 or 6 paddles just to actually start moving due to the speed of the water being sucked up the face.
- dont think starting further back and trying to catch the wave extra early is the answer. Usually that means you get hung up at the top of the wave. Have a look at the pros - they pop up 1/2 or 3/4 up the face, not right at the top. steveylang hit the nail with what often happens to beginners. For a steep wave, that might mean positioning yourself in a spot that is almost under the lip rather than out the back. The other thing that can happen is people stay a bit further out so they can get over the larger set waves, and then just miss the waves they go for because the smaller waves are breaking just that little bit further in.
- dont stop paddling and try to pop up too soon. Maybe you need to push your weight forward (eg bob your chin down to touch the board) and sometimes you will need to pull your weight back. But you need to have the board sliding down the face. It feels weird, wrong - your brain doesnt like being at an angle especially a downwards angle. But if you stop paddling or slow down now, then something is going to go wrong
In short: get your positioning on the board and paddling right. Play around with where you are starting in the line up and when you start paddling. Keep paddling until you feel gravity taking over. Accept that it feels a bit scary. (although, as a guess without any actual evidence, I bet you are starting too far back and not paddling fast enough to get to the right position/catch the wave)
Admittedly some waves are much harder to catch than others (low tide shore break requires very very fast and accurate timing, for example). Some waves suit some boards more than others (eg longboards on small steep waves can be hard because there is no where for the board to fit); but that said this is only a question of degree. Most boards can catch most waves - so its the surfer lacking the skills
For what its worth, I think everyone goes through this phase. In a year or whatever you will be catching these very same waves without even thinking about it. If you can get a video that would be great - we can get stuck into all of your flaws...
