Why cant i fly in Minecraft?

Sometimes lag causes the timing between the two space-bar clicks that makes you fly creates too much time in between the key clicks so instead of double clicking space to fly, the lag makes it register as only a single click. Of course, like the double-block placing lag type, this kind of lag can happen even if you have the best graphics card. Just keep trying to double click space at different speeds and eventually you'll fly. It's annoying, I know. But there's really no way to fix it.

However, I know Optifine has been known to worsen the double block lag, so maybe uninstalling that will fix this similar problem.