Okay, paella. One of my favorite things. First, paella works best with the correct sort of rice. Best is something like Bomba, a medium grain rice capable of absorbing a lot of liquid. But you can do it with long grain rice, too, with modification. One of the benefits of Bomba is that toward the end of preparation, when you stir in the rice and then the liquid, you don't have to worry too much about adding too much liquid. Bomba will take up a good bit of extra water. No cover with paella. And a rice crust on the bottom is no sin. It can be a prized part of the dish. An no stirring after adding the rice. Just let it cook. It should cook at a good honest simmer. Simmer, not steep.
This next part is common practice, and I find I need to do it this way. Some people seem blessed and able to do it differently, but I have just the problem you experience if I don't do as below.
For flavored rices such as paella, Spanish rice, etc., using long grain rice, you need to do things a bit differently. Essentially, you saute the rice until be begins to visibly brown. You will see it first become translucent except for the core, and then brown slightly. How brown you can tolerate depends on the dish. Quite brown for Spanish rice. Less for lighter colored dishes. Then, you add flavors, etc. You can saute it with other sauteed ingredients. Then, simmer as with paella above. You may get even better results by soaking the rice for an hour first. And a little refinement that can help is, once the rice if nicely browned, hit it with an acid, lemon juice, white wine, etc. Seems to set it up to be more absorbent. I do that even with Arborio rice for rissoto.
And what I think is an important point, with any kind of rice that you saute first. When you add the liquid, water or broth, make sure it's simmering hot when you add it.
But for best results, a rice of the correct characteristics for the dish should be selected from among the great many varieties. They vary greatly. Most places in the US, you can at least find Arborio. It's a short grain, but it has great powers of absorption and can perform well for rissoto (which it's specifically for) and paella.