Hi Laura,
Welcome to the Moz community!
Without a domain to do some further investigation, these types of questions can be some of the toughest to answer because there are so many variables that could be causing the problem.
I can't comment on your current site but what I can say is that from experience, if this is happening it's typically going to be one of three things (maybe a combination?):
- More links pointing to the individual product pages than to the category pages
- Either no or low quality content on the subcategory pages; basically just showing a big chunk of products and nothing else
- Poor internal linking with more internal links pointing to those products, the category pages in a separate section of the nav, no category breadcrumbs etc. URL structure ties in here in much the same way.
If we think about typical user behaviour, it makes perfect sense that your product pages would be getting more backlinks than the categories. If we're talking about real users in a forum and they want to reference a product you sell, they're going to link straight to that product and the same can be said in most other contexts too.
This isn't a problem at all - real backlinks being earned in a natural way is always good. It just means you may need to put some effort into driving strength to the category pages too.
Content is a topic that gets so much attention in this industry that I don't think I really need to elaborate here - just make sure there is a decent amount of high-quality content on the pages, styled in a way that doesn't bury the products. UX first!
Finally, the internal linking. It's very hard to give advice on this part without seeing the site but basically, search engines tend to look at your nav to get an indication of which pages are likely the most important on the page from left to right, top to bottom. If you have an unusual structure where the category pages are buried down in the drop-downs (or - gasp - not even in the nav because there are buttons on the home page or something!) then you're missing an easy win there.
Don't forget that breadcrumbs and URL structure should ultimately match this same layout. Something like website.com/category/product.
This is all quite vague but hopefully it makes sense to the site you're working with now. Always happy to take a closer look if you'd like to share the URL here or in PM but I completely understand if you're not comfortable with that too.