Trouble with iMac connecting to macbook over WiFi

Here is my scenario.

I have an iMac that is connected via ethernet cable to Time Capsule. It would appear the iMac does not see the MacBook Air that is connected over WiFi. And vice versa. The Air does not see the iMac either.

I have a NAS which is plugged into the TC with ethernet cable. The iMac connects to it fine. The Air cannot connect to it.

This used to work. I have not come across this issue until now. But I am guessing it had something to do the TC.

Is there a setting that needs to tweaked in the TC to allow this connectivity of hard-wired devices to wireless devices?

Thanks and I look forward to your reply.