new iMac not connecting to Time Capsule

I bought a new 27" iMac with Retina Display this past December. I installed the same programs I had been using on my old 2009 iMac and wirelessly connected my Time Capsule (2 years old). It backed up once and then I began to receive messages telling me "backup disk not available" and warnings that my computer has not been backed up. I opened Airport Utility on my iMac and was only able to see the internet - not my TC or Airport Express. I can view my TC and AE via my iPad and iPhone as well as my old iMac (running Yosemite). I finally called Apple in late January and they had me reset TC multiple times. Nothing worked. They suggested I take it to the Apple Store and let them look at it. So a few weeks later I took the TC in - and it worked like a charm with any computer they tried. I brought it back home and hooked it back up and it worked - ONCE! Since that one backup in February I have the same error messages as I did previously and neither Airports are viewable from my new iMac. Any ideas? Apple Genius guy said it's probably my iMac and to get it replaced - which is easier said than done since I use it for work everyday.