You’ll have to keep trying. The jump is wider than it looks, and you have to jump at the very last moment to cross it. However, if due to some very strange glitch you really are falling through the floor and you’re using the PC version of the game, you could use the noclip mode to pass that single screen, then continue from there normally. If you want to try this, search the Oddworld Discussion forum for the thread entitled ‘-it_is_me_your_father’.
For future reference, you should take care to post threads in the right part of the Oddworld Forums, where they’re more likely to have the right kind of readers. Technical and gameplay troubles should be posted in the Oddworld Help forum (to where this thread will be moved). Also, it’s really helpful if you’re asking about gameplay to quote the screen code that uniquely identifies where you’re having difficulties. Just pause the game and jot down the code that should be something like ‘F1P01C04’.
Good luck with the jump!