Either your router or a firewall running on the machine may be blocking some traffic needed to receive a call.
Thank you for your answer. I did consider this but it is a new problem for me. I haven't changed router, router firewall or Mac firewall settings. I wondered if it is to do with the last software upgrade for ipad and iphone that has created problems. I have no facetime issues receiving calls on my ipad or iphone -(same router) - just the Mac.
I have checked for updates for my Mac but seem to have everything up to date.
I really can't work it out.
Thanks again for getting back to me.
I am sorry I didn't get back to you but I have been away for a while. I have discovered that when I turn the firewall off on my imac I can receive Facetime calls - turn it back on and I can't. Seems a little silly that Apple's own firewall blocks Facetime.
I wonder if anyone else has this problem. I am a little concerned about having to keep my firewall turned off.
Thank you very much for your help anyway.