Why are my Shopify collection pages not indexed?
Usually because they are thin, duplicated by filters and variants, or canonicalised away. Shopify generates collection and filtered URLs automatically, and Google declines most of them. That is expected. The problem is when the collections you want ranking are caught in the same net.
Separate the noise from the loss
A large not indexed count is not automatically bad. Sorted and filtered variants of a collection are supposed to be excluded. Open the reasons breakdown and look only at the collections that carry commercial intent.
Give the page a reason to exist
A collection that matches a real search, with its own copy, its own internal links and a genuinely different product set, gets indexed. One that is a filtered view of another collection does not, and should not.
Then link to it
Internal links are how a new collection gets found and how its authority becomes visible. Collections orphaned behind a mega menu dropdown are invisible in practice.
Real numbers from a live Shopify client account. Every result page carries the before, the after, and what the figure does not prove.