Intermittently Not Recognizing Keyword

I recently bought a new iMac27. Since I prefer a wireless keyboard, I connected the wired keyboard that came with the iMac27 & connected it to the older iMac21. At times when I awaken the iMac21 from sleep, there is no response from the keyboard until I disconnect & reconnect it from the USB port.

Any thoughts as to what might be causing this? Thank you so much.