Here's another thing that may be affecting your rolls. I find that the higher one jumps from, the faster he must run to execute a good roll and transfer momentum forward. I think that in order to roll, your forward momentum must approximately equal your downward force. For example, you should be travelling at close to or slightly greater than a 45 degree angle as you go down. That's why when you are above about a 4 foot drop, you can't roll without a running start, because you just don't have enough forward momentum to lean into a roll. If you are higher than that, and must leap from a stand, do so straight down without a roll because if you force the roll without enough forward momentum, you will either (1) collapse too quickly and downwardly, resulting in you striking your shoulder or chest harshly, or you (2) won't have room to bend your knees enough as you lean into the forced roll and land straight legged (very dangerous for joints and bones), or finally you will (3) land and absorb most of the impact in your legs and force a redundant, useless roll. I should write a full article on this with diagrams, but basically you need a good running start for drops higher than 4 feet if you want your roll to be effective. And I'm talking about experience here, because I tested this when I was under the impression that you must always roll yesterday, and I experienced all these things without any real injury, just some minor soreness in my knee later that day. Don't fear about an 8 foor jump straight down without a roll if you've trained the muscles in your legs to quickly absorb impact. Practice low and move progressively higher as always, same with jumps with rolls.