You're competing for words nobody searches
Product pages target the narrowest searches there are: the exact product name. The trouble is barely anyone types 'Draft Ultralight 4/3 Chest-Zip Wetsuit Size M' into Google. They search 'winter wetsuit' or 'chest zip wetsuit', and those are category searches, which is collection-page territory, not product.
So a product page ranking for its own long name is easy but worthless, and ranking for the broad category term is nearly impossible because the page is too specific. That mismatch is why most product-page SEO effort quietly evaporates.
Manufacturer copy is duplicate content
If your descriptions are the same block of text the manufacturer hands every stockist, Google has seen it on fifty other sites already. There's no reason to rank yours over a bigger retailer running the identical words.
Rewrite the descriptions on your important products in your own words: who it's for, how to choose it, what's genuinely different about it. Even a few original, useful lines beats a copy-paste spec sheet Google has read a hundred times.
Thin pages with nothing but a spec table
A photo, a price, and a bullet list of dimensions is thin, and thin pages don't rank. There's nothing for Google to understand the page by beyond the title.
Add real content to the products you actually want to rank: a short buying-context paragraph, an FAQ answering the questions people ask before buying, care or sizing notes. It helps the page rank and it helps the visitor buy, same effort, two wins.
No product schema, so no rich result
Structured data is what puts your price, stock status and star rating into the search result itself. Without it you're a plain blue link sitting next to competitors showing a photo, a price and a row of stars. Even when you do rank, you lose the click.
Most Shopify SEO apps add Product, Offer and review schema in a couple of clicks. If your products have reviews and none of it is marked up, you're handing the click to whoever did bother.
Orphan products nothing links to
If a product is only reachable by scrolling a collection grid, Google reads it as low priority. Products with no internal links pointing at them, from collections, from related-product blocks, from the blog, get treated as an afterthought and ranked like one.
Link to your hero products deliberately. Feature them on the homepage, cross-link related products, mention them in relevant blog posts. The products you most want to sell should be the most linked-to on your own site.
Where the effort actually pays
Don't try to rank every product for a broad head term; you'll lose. Win the collection pages for the category searches, then let product pages do two jobs: convert the traffic the collection sends them, and pick up the long-tail, specific searches ('4/3 chest zip wetsuit womens') where there's real intent and less competition.
So the order is: fix collections first, then on your top products rewrite the copy, add schema, thicken the thin pages, and link to them properly. That's product-page SEO that earns its keep instead of just feeling productive.
More on ranking and on-page SEO: Why your Shopify collection 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.