Ok, my thoughts are...
In the US, I think 'Like a Prayer' actually caused some damage to her career, despite being her signature song (IMO). She'd just come from a diamond album (True Blue), but 'Like a Prayer' sold half of what that did. This was the first time Madonna had really been controversial and she offended a lot of people with that video. I know this is a really unpopular opinion because the single was so huge, but I do wonder, if she released a safer song and video, would she be more
popular in the US?
Obviously the song didn't end her career, because she still had great success after it (including some of the biggest hits of her career over there). But she definitely lost a lot of fans, which is why 'Ray of Light' couldn't pass 4 million units sold. It was released in 1998 when CD's were selling like crazy, and it was critically acclaimed and heavily promoted. If Madonna was really still as popular as she was in the 80's, that album would have sold over 5 million copies easily (IMO).
Anyway, 'American Life' was a huge error in the US. I personally think it's one of her more interesting records, but the country was not ready for that album I think, and I guess I sympathise. After all, Madonna is a singer, not a politician, so the eye rolling at 'American Life' was understandable.
And the fact that Madonna barely promotes her work in the US has definitely not helped matters at all. Her sales could be a lot higher if she did. So that's definitely one of the major factors (not only in the US, but worldwide).
'MDNA' hasn't done well in the UK which is unusual because Madonna's name alone still sells here (or it did). I think it's all going wrong for Madonna here because of her image, and this is applicable on a worldwide scale too. More than ever, there seems to be a focus on her age. When she was with Guy, nobody really focused on this that much because she was a normal, married woman and was ageing quite respectfully. But then in 2008, with 'Hard Candy', she worked with Timbaland and Justin Timberlake and this was such an obvious attempt at trying to stay current. I don't think there is anything wrong with trying to stay current, but many other people do (when the artist in question is 50). So that album wasn't very popular but she made matters a whole lot worse with 'MDNA' and I actually think this is where it has all gone wrong for Madonna.
MDNA is all about Madonna trying to be young, and this time the comments about her are justified. She is dressing like a young woman, she is dating a 24 year old (which I find really creepy, she is old enough to be his mother), she is still releasing music which follows the trend and now, she has resorted to crude and unpopular publicity stunts to try and stay relevant. It sounds like a car crash and this era has been just that.
Madonna can still rectify this. People want her to age appropriately. They've made it clear they don't want to see her ageing body and they don't want her to follow the current trends. If she fixes these problems, then she can have another hit album. But for now, she just seems to be irking a lot of people by doing the complete opposite.