Apple Keyboard with Numeric Keypad stops working

[Nick Richards]

Hardware: 2014 MacBook Pro & 2014 MacBook Air, both*OS X*Yosemite

From time to time my Apple Keyboard with Numeric Keypad stops working.
If I switch USB ports that sometimes fixes the issue and it comes back to life.
Other times it doesn't.
The same issue happens on my other mac with different keyboard.
It happened pre and post OS upgrade.

Any ideas of what the cause could be?
Is it a software conflict?

Help??

 

[harryb2448]

Any spills or moisture on the keyboards? No USB hubs involved?

And like mice they do wear out.