I went through a phase about a year ago where I managed to convince myself that I had absolutely nothing to write about. And then, even if by some miracle I did have an idea, I told myself that no-one would want to read it if it was written by me as I didn't have any qualifications in that area and I didn't know a lot about the subject. Then I had a more positive thought: do you really have to be qualified in or know a lot about the subject you are writing about? Well, it can give your writing more credence if you tell the reader all your relevant credentials and experience in the field you're writing about but it won't stop you from getting published if you're not the expert. You can, of course, always get quotes from experts to put into your piece to give it some oomph and give your writing some authority.
Take the example of the the archery article I researched, wrote and got published (twice!) Did I know a lot about archery before I wrote it? No. Was archery a hobby of mine or did I compete? No. But I knew of people out there who did so I asked them. I also did a lot of reading up on the subject on the Internet and watched programmes on the TV about it. There really is more than one way to skin a cat (hate that expression but you know what I mean.)
So, if you're stuck for writing material or want to expand your repertoire then don't be afraid to try something new. Choose a subject that interests you but you know little about and explore that. You don't have to restrict yourself to a couple of subjects you always write about. In fact, doing that can spell disaster for the writer as publications go bust or you run out of things to say on those subjects - unless you can go at the subjects from a different angle. Cast your nets wider and see what you catch.