Without seeing the site you have it's hard to tell you what I think, but maybe it just comes down to your content and the text you use to target your audience? Are you using UK English and terms throughout? That could be an issue. It would be wise to do some research into language used in the US and how it differs from that in the UK.

A good example of this is that in the UK a lot of gay spanking content involves "chavs" and "scally" boys, or the longer fiction they use for boosting SEO is all about boarding school etc. None of that is really going to resonate with a US audience. If you're using keywords like "chav arse spanking" you're not going to get many US hits because "chav" is a British slang term and "arse" is a UK spelling.

If you want to improve US search, I would suggest creating more content targeted to a US audience, perhaps in the jock and frat area, which is the most common gay spanking related scenario in the US (in my opinion).