sending email contact details not autofilling

I'm using Icloud for my contacts, and when I go to send an email from my IMac I used to be able to start typing in the (to Address) and autofill would start to complete the details for the email address. This has stopped for some reason and on both the IMacs I have the same problem and have to manually type every email address in by hand! why has it lost the conection to Contacts?

Thanks in advance to anybody that can help me solve this one.

Mark