Think of books aimed at the youngest children. They're about problems like bed time, and the arrival of new siblings. A little bit older and the problems are about going to school or losing teeth. A bit older, and the problems shift to friendship groups and independence. Teenagers' problems are around things like peer-group pressure and sexuality. The problems are relevant to their readership - not many teenagers are worrying about paying their mortgage, not many pre-schoolers are thinking about their exams.
It's the same with adult fiction: the problems need to be relevant to the readership. Joanna Trollope's The Rector's Wife, featuring a middle-class heroine who took a job stacking supermarket shelves to make some money, became a best seller in a recession. The Shopaholic series by Sophie Kinsella became popular in a time when the economy was booming, and spending £1000s on shopping was fine. It seems out of date in these more austere times - I expect there are many books in the pipeline where redundancy and financial problems are central issue.
And of course there's the fantasy element, so the James Bond books featuring foreign travel and the high life were written at a time of austerity when travel abroad was expensive and difficult, and I can't help but suspect that the popularity of the Twilight series in part is down to the sexual pressures on teenage girls and young women today.
Problems don't have to be directly relevant - for example, not many of us live in a stratified society with limited life choices as depicted in Jane Austen's novels, but most of us still live with a limited social circle where we hope to make a good choice of a partner.
The more relevant the problem, the wider the readership. Most people aren't going to be interested in how I'm solving some structural problems in my current novel so I haven't said anything to my friends and family about it, but I have written about them on this blog as I think solutions to writing problems may interest my readership here.
Think about the problems your characters are solving in your writing, and work out how relevant they are to your readership. And if they're not that relevant, then now's the time to make them so.