There are three factors that might cause this.
Lack of Paddling Speed
If you are moving too slowly, you will be hit by the wave rather than catching it. You should be feeling like you are joining the wave. This will require some time to build up paddling strength and technique. The last three strokes need to be big. Stretch out long in front, submerse your arm nearly to the elbow and pull back all the way. Beginners typically stroke too short, and only with their hands. Lack of speed at takeoff is likely the main problem.
Sometimes you will miss a wave because you are slightly too far from the shore. Other times, you'll be too close. This can cause a nosedive as the wave is too steep at the point you are catching it. Practice by looking at waves and predicting exactly where they will break. You should not be hit by whitewater as you catch the wave, but it needs to be steep enough to move you forward.
Miss it: too far out.
Hit by whitewater: too close to shore.
Your board needs to be horizontal as you paddle. The nose must not be sticking up in the air. If it is, the tail will be dragging too deep in the water. Nearly everyone gets this wrong initially. When your board is angled, you can not obtain enough speed. As a result, you will be almost stationary as the wave tumbles. Instead of picking up speed and sliding down the face, you are rotated forwards by the wave.
Nose in air while paddling: too far back.
Nose under water: too far forward.
Nose should be on the surface.
When a surfer jumps to their feet, the weight shifts backwards considerably. But this happens as the board is starting to plane. They are not in that position they catch the wave. You need to be central on the board. Once you feel it start to accelerate, you can slide back and remain lying down.
Each of these things plays a part. Improvement in any factor will increase the number of good waves you catch.