I have a brother who moved from the UK to Spain some years ago, and has now moved back. He and his wife are of retirement age, and although there was a lot they did like in Spain, they never fully settled.
Why is it some can, but others can't, settle in another country? Well, there are of course lots of reasons why some expats return to their homeland. And a reason one person will give might not apply to the next person.
On some forums I've read that boredom is the number one reason why some have gone back to the UK. Not being used to the heat in Spain means that when the sun is shining and it gets very hot the only place to be is inside the home. But staying cool for several hours a day for weeks or months on end and not venturing out until the evening has its downside - boredom! Worse still for those who have young children.
Another reason some give is the feeling of detachment, especially if you don't have many English-speaking people living close to you. Although some expats make the effort to speak Spanish it often doesn't get past being able to ask for drinks and food along with the customary "hello" and "goodbye." Being fluent in Spanish is another matter, and that makes it difficult to get onto a deeper level with the Spanish people themselves.
Some are anxious about the possibility - or as they get older the likelihood - of having to spend time in hospital. A worrying time anywhere, but especially in a foreign country.
The poor exchange rate has meant the pound doesn't stretch as far as it used to. Those with fixed pensions paid in sterling are getting fewer euros, therefore they have to tighten their belts much more. And, as employment prospects in Spain are not good right now Brits who were in work, especially in the building trade, are finding they don't have the work they once did.
And finally, the pull of home is ever present, as homesickness affects some. They miss their families and friends, the British way of life, even the weather. And for them that's good enough reason to put their property up for sale, pack their bags, and return home to the UK.