I've only ever heard one once where I live in Gloucestershire, which is odd as it's a very rural area. But I do hear them every year where I work in Wiltshire. They certainly do seem to be on the decline.
I've also seen a theory that they are arriving too late. Our birds have responded to warmer (it says here) springs by nesting earlier. By the time the cuckoos arrive the eggs have hatched so the cuckoos cannot add another one.
Give them a year or two and they will probably learn
I heard one last year but nothing yet so far, I'm not sure why I don't hear them anymore but I have noticed that the rhines seem to be cleaned more often in recent years thus destroying the reeds growing on the banks where I think they used to hang out, so that could be a reason.
They didn't arrive late this year, they arrived early - one was heard by ornithologists in February! But according to the RSPB, cuckoos are among the eight species of birds that have declined most in numbers in the UK over recent years.