The key to getting a waterproof seal on a dry bag is to ensure that the first fold is as tight and crisp as possible.
It is this fold which creates a waterproof barrier to stop any ingress.
Once you have ensured that fold is as neat as possible then it is also important to keep folding over and over so you have a series of very tight bends in the material which are adding compression to all of the other folds inside of them.
With a lightweight dry bag material sometimes it is necessary to have a stiffening bar or section of webbing at the opening of the bag in order to help make that first fold as crisp as possible.
And conversely with heavyweight bags where the material is much harder to fold it is more important to ensure that that first fold is as tight as possible.
We see many pictures of people who have loosely rolled a drybag closed. That will cause leakage if the bag is submerged for too long.