Collections are your money pages
A collection page targets a whole category of buying intent at once. Rank one and you catch everyone searching that category, then funnel them to the right products. That's far more valuable than ranking a single product.
So collections should be your first SEO priority, not an afterthought. Most stores have this backwards.
Reason 1: the page is just a product grid
If your collection is a wall of products with no words on it, Google has nothing to read and nothing to rank. Thin pages don't rank, full stop.
Add a couple of paragraphs of genuinely useful copy: what the category is, how to choose, what you stock. Above or below the grid, written for a human, not stuffed with keywords.
Reason 2: brand-only or default titles
If your collection titles are just your brand name, or worse the Shopify default, they can't rank for the category. The title needs to say the category: "Merino Base Layers | Your Brand", not "Your Brand".
This is the same disease that keeps whole stores invisible. I've torn down a store whose every page was titled 'My Store', and it's worth seeing how badly that plays out in search.
Reason 3: nothing links to them
Internal links tell Google which pages matter. If your collections are only reachable from a dropdown menu and nothing else points at them, Google reads them as low priority.
Link to your important collections from the homepage, from related collections, and from blog posts. The pages you most want to rank should be the most linked-to on your own site.
Reason 4: duplicate and junk collection URLs
Auto-generated tag collections and faceted filter URLs (colour, size, price) create dozens of near-identical pages that compete with each other and bloat your index. Google ends up unsure which one to rank, so it ranks none of them well.
Keep a tight set of real, intentional collections. Stop filtered and tag-based URLs from being indexed (canonical tags or no-index), so your effort concentrates on the pages that matter.
Fix order
Title first, then real copy, then internal links, then clean up the duplicate and filtered URLs. That sequence puts the highest-leverage fixes first.
More on ranking and on-page SEO: Why your Shopify product pages don't rank, and how to fix them · How to write Shopify meta titles and descriptions that get clicks · Why your Shopify blog gets no search traffic · Are out-of-stock products hurting your Shopify SEO? · Why your review stars don't show up in Google · Why your Shopify store doesn't show up in local search · The Shopify SEO checklist that actually moves revenue.