There are often banks of slots without camera coverage. In Nevada, even if the casino does not do it on their own, the state generally requires camera coverage on linked progressives like Megabucks. I don't know what the state requires and what casinos have internal policies on their own, but generally high denom slots or anything with a big jackpot (an in house progressive) has fixed camera coverage which means the machine is being filmed all the time.
Many of the cameras in casinos are pan/tilt they might be able to pick up a particular slot machine if aimed, but there are machines that are not on constant 24/7 coverage. Actually that would be most of them. The areas around them might have fixed coverage, but not good enough to see the results on a specific machine.