Hmm. I think it's a valid question, but there are couple of problems with it.
First of all, what do we mean by 'adjust'? There's the functional level (able to get about, understand the locals, use the money, etc.), but then there's the emotional level (level of satisfaction, lack of homesickness). Most often when this question is asked, people seem to be focusing on the second. And, although I realise that the thread title is meant to be light-hearted, I think we need to be careful about the implication that the folks who have an easier adjustment are somehow
better than folks who struggle.
I do think that some people end up landing in a lot better situations than others as well - what your job situation is, how much money you arrive with or are able to make, do you already have connections here or not, perhaps what your partner does or his/her job opportunities, good (or bad) in-laws, what area/neighbourhood you end up in - so many different factors.
This! Oh, my God, this! It makes a huge difference. There's a quote often misattributed to William Gibson (no idea of the original source) that I think is relevant here:
"Before you diagnose yourself with depression or low self-esteem, first make sure that you are not, in fact, just surrounded by a**holes."
The most prepared, easy-going, flexible, realistic person in the world can hit a brick wall if they land in the wrong situation. That's not to say they won't be able to adjust eventually (or, more likely, find their way into a better situation), but I'm uncomfortable with what I sometimes feel is the assumption that, if a person is struggling after their move abroad, that it's because of some failure on their own part. In the interests of full disclosure, I'm sure I feel this way largely because my own transition hasn't been perfectly smooth.
However, in terms of personal circumstances that have made things more difficult, I would say it boils down to the strength of the ties I have back home. My family is amazing. There are scores of them, and I grew up feeling close to each one, and I miss them. There's no getting around that. No matter how well things are going for me here, I will always miss them, and miss being able to share in each others' lives.
Beyond that, I genuinely liked the area I used to live in. I've lived and worked in several different areas of the U.S., and been abroad, and after going back to Wisconsin, I could honestly say that it was my favorite. Sure, there were down-sides (winter, mainly)(actually...just winter), but overall, it had everything I wanted-- except my husband. So obviously, moving to a new place-- whether in the U.K., or just to a different state-- comparisons will be made, and the bar is set awfully high.