In some circumstances (such as if your game is lagging out really bad and some other times), you can't use double-space to begin flying because the game does not recognize you pressed space twice and just makes you jump once or sometimes jump twice. It could also be an issue for people who are unable to doubletap space quickly.

This is much more of a problem in 21w08b because it is very laggy due to MC-214883, but it has existed for as long as I can remember.

For anyone else experiencing this issue: A workaround is to hit F3+N twice as it always makes you fly. To stop flying, F3+F4 to survival mode and then F3+F4 again.