When in play mode, arrow keys don't advance slides in Keynote. Any ideas?

Try resetting Keynote as follows:

Perform all the steps in the order listed

To uninstall Keynote, you must use an application removal tool for this procedure to work

Appcleaner is free and works well for this procedure, it can be download from here: Appcleaner

• uninstall Keynote
• empty the trash
• shut Down the Mac (Apple menu > Shut Down)
• wait 10 seconds, then press the power button
• immediately after you hear the start up tone, hold down the Shift key
• release the Shift key when you see the grey Apple screen, start up will take a few minutes longer than usual as the Mac is performing a software repair
• after the repair and the Mac is fully booted, restart the Mac normally, (Apple > Restart)
• install Keynote from the Mac App Store

Unfortunately this didn't help me. The app you suggest wouldn't actually access Keynote to delete it.

Anyone have any other ideas? I was literally working 5 hours ago, and now it will work once, then stop. I've deleted the application, rebooted my computer twice and downloaded the latest version of Keynote from the App Store.

I had the same problem, and didn't feel that any of the solutions were credible. Playing around, I pressed the left of the mouse pad, and the slides started advancing. I then tried the arrows and they worked too. I find that I have to do the same every time I start a Keynote presentation. Obviously Keynote does not recognize the arrow keys, until somehow the mouse introduces them to each other. Remember when Apple used to make fun of Microsoft for glitches?

Keynote worked fine on my laptop until today. Your tip worked for me. Thanks so much!