Q: Air Drop : what Sharing preferences need to be selected?

In Air Drop, what Sharing preferences need to be selected?

My 2010 MacBook works great and I see my 2 iMacs in AirDrop BUT my new 2015 MacBook with updated OS X Yosemite does not see any of my other Macs.

My iPhone appears .. that is all.

The other Macs appear on my 2015 MacBook in the  Finder sidebar shared computers .. so why not AirDrop?

Thanks.

Michael

I looked at

https://support.apple.com/en-us/HT203106

plus other threads but still cannot see other Macs.