For Sentry/Hulk: It could be explained one of many ways.
Firstly, obviously Hulk's energy is transfinite, and presumably, so is the Sentry's (though, that sort of retcons how he gets his powers and such, but hey, wouldn't be the first, won't be the last.) So that means that while he has TONS of energy at his disposal, and that it CAN recharge, even midfight, it does expend and "run out". That, or he just craps out randomly, or he has some odd absolute limit that the Sentry found.
The Sentry either has transfinite power himself (the most likely case, as his power originally was dependent on light to excite his powers to begin with, meaning they have a beginning, meaning they have an end, sort of.) Or he has infinite energy, and his mind is just set so he can't activate his powers without sufficient light from a certain time frame, or his powers crapped out (which is more likely than the Hulk, due to his instability.) They crapped out comes from just how MUCH energy he expends, and can viably handle without going overboard, assuming that by restraining Void level powers, he may have just mentally shut down. We DO know that the Sentry's powers are literally based on his mind, and what it can do, as represented by the Void, and the odd powers the Sentry has shown to have in passing. This may be affected by the many things that are in the fight, knowing he is already unstable as it stands. The Hulk is/was/can be his friend, and probably doesn't want to go about killing him. He might be aware of the Void's level barrier, and is trying to avoid it. He might be stressed out over all of this to begin with, and is having a hard time accepting that the guys he's backing just launched one of his old friends into space. Many things might be going through his mind. The Void may simply be taunting him from within, and was losing control. Rob is all about control, and the fail thereof. He might have simply not been able to hold his power. But it's most likely, that the Hulk requires so MUCH power, that the Sentry can't maintain without the negative side effects coming forth.
And the same can go for Hulk. I mean, the Hulk isn't exactly the picture of mental health, but more of mental conflict himself. And the same can easily apply.
While I think neither is really the case, and simply the Sentry probably got overexcited, and burned himself out, or shut down before he couldn't stop, and I think the Hulk probably just maxed out, or may have been affected by the Sentry himself, as the Sentry has shown that ability, amongst similar abilities before.
