OSX 10.9 can't connect to OSX 10.11
I have two identical 27" iMacs on my desk, and a Mac Mini. The Mac Mini and one of the 27" iMacs is running 10.9.5. The other iMac is 10.11.
The 10.11 iMac can see the other two computers as servers, and can interact with them – Share Screen and so on.
The two 10.9.5 machines can see each other, and interact, but they cannot see the 10.11 iMac.
Can anyone suggest how I can get the 10.9.5 machines to see the 10.11 machine?
iMac 27″, OS X 10.11