My Macbook Pro cannot see my Macbook when using Wifi

I have a 2009 I-Mac that is hooked to the Ethernet. When I hook my 2012 Macbook Pro to an ethernet cable I can see the I-mac. However when the macbook pro is using wireless (no ethernet) it cannot see the I-mac and the I-mac cannot see the Macbook. Both machines are running Yosemite. I have security disabled on the Linksys router. All sharing on both Macs is on. I don't have a great deal of computer experience so I am sure there are many things I have not tried.