First, we have to assume you've got the "right" mask, the one that mates to your facial features and contours well, as in the best possible. But the possibility is that you don't have the right mask is out there. Right mask? OK, moving on.
Right size is either yes or no. No means get the right size, a yes then moving on. Unfortunate you're not likely to know if the size is right unless you've got at least 2 for comparison sake. IMO one will prove better than the other.
Another variable is if your choice is an F20. (maybe other masks are like this too) You need to choose cushion material, AirFit is silicone, while AirTouch is memory foam. I believe they are shaped a bit different too. AirFit being a bit more blunt at the top, where the AirTouch is more pointed and elongated in the nasal bridge. That's what I observe when I see them side by side in the same size I own.
My F20 example on myself is silicone works great but memory foam doesn't seal as well.
For both styles of cushion material, I've tried more or less every bit of those Velcro strap adjustment ranges to get it to work, and I did eventually get there. I've tried tight to too tight to very loose to somewhat loose. Even combo loose and tight, meaning top vs bottom straps and vise versa.
Another variable is exactly where on your face do you park it. High or low on the bridge of your nose makes a difference, at least on my F20.
For some, there's no leeway. It has to be just so or you'll have a leak no matter what.
If after what YOU consider reasonable time and effort, if you find you cannot get the leaks to stop, it's probably time to start over with another mask.
YMMV! But that's how I got my mask to not leak.