I just got hold of this series from Network since they recently rereleased it. I'm enjoying it, the main couple are really warm characters and I laughed a lot, especially at the great Joan Sanderson's acerbic asides. Funny that it never gets brought up in conversations about older sitcoms. Yes there are a few jokes that look questionable through a modern lens but nothing outrageous.
I remember it being on and to answer your question I think it suffered from not having a lead character that's known to the audience. Casting a favourite actor as a main role player has always been a key way to pull viewers in to any sitcom, however good the content is. You look at that picture and you'd struggle to recognise the two leads, let alone name them.
Added to that I believe it was a light domcom in an era stuffed to the rafters with them, so it was hardly a thing millions of viewers would be lured to watch.
