I don't know what else to tell you. The zooming function works on my iPad as I described it above: I pinch to zoom in and out. In addition, the yellow slider line also works for me.
Perhaps you need to fully close the Camera app and then either restart or reset the iPad...or both. To do (apologies if you know):
Fully Close an App: Double tap the Home button (or do a 4-finger swipe up) to reveal a row of icons and cards in the multi-tasking viewer. Swipe up on all the cards until they are all gone.
Then...
Restart: Hold down the power button until the power off slider appears. Slide it off. When the iPad finishes shutting down, hold down the power button again until the Apple logo appears.
If that doesn't help, try this last step...
Reset: Hold both the power and home buttons until the screen goes dark and the Apple logo then appears. Once the logo appears, release the buttons. After a time, your homescreen will appear. Please note that it may take some time for the Apple logo to appear or for your homescreen to appear (or both). This is normal. Eventually, the homescreen will appear.
Hopefully. That will help. Good luck.
Marilyn