Boy have I heard this one before, and even said it myself! There are so many suggestions when your cat has litter box issues: take of the top, put a top on, change the litter, change it again, move it, add one, divide by three, do a dance, say a prayer, bury a statue in your yard .... you get the point.
When one of my cats had litter box issues I tried all these things and more. I truly tried everything and nothing worked. The problem was, I tried everything in an overlapping, undocumented way. If there was any slight improvement, I had no way of knowing what worked and when it worked.
Then I made a litter box cafeteria for my cats. Using isolated variables and documentation, I was able to find out what worked for them. It was time consuming and expensive, but so much better than cleaning up accidents! It was interesting, too, as I learned about my cats preferences and watched them "vote" in each box.
I've published this method in a downloadable, printable document and it is available here. The Litter Box Cafeteria Kit is my go-to resource for house soiling issues, and it's available on its own or included with a litter box issues behavior session.