Why can't I see all apps when I swipe up on my iPad?

Are you seeing the App Switcher? If your iPad has a Home Button, then try double clicking the Home Button. You do need to pause while holding your finger after swiping part way up from the bottom when using the swipe method.

Switch between apps on iPad - Apple Support

To expose the App Switcher page, swipe-upwards from the bottom edge to the centre of the screen, pausing briefly before lifting your finger from the screen; you will see a thumbnail image for each of your currently open Apps.  If many Apps are open, swipe to the left or right to see more.

Simply tap a corresponding thumbnail to switch directly to the App. Should you need or wish to do so, Apps can also be force-closed by swiping the corresponding thumbnail off the top of the screen.

Additional information can be found here:

Switch between apps on iPad - Apple Support

Swipe up and hold your finger on the screen to bring up the App Switcher.