You're right, Amazon's ad algorithm strongly favors top-selling ASINs within a variation group. It's not a "penalty," but an optimization for sales velocity and conversion, as those ASINs historically generate more revenue for Amazon.
To get more balanced delivery, you need to "force" visibility:
Proven Strategy: Separate Campaigns/Ad Groups for Underperforming ASINs
Isolate Underperformers: Create new, dedicated Sponsored Products campaigns or ad groups for your 12-13 less visible child ASINs. Each of these ad groups should contain only one specific child ASIN.
Target Aggressively:
Use manual keywords (broad, phrase, exact) relevant to that specific ASIN.
Implement Product Targeting to show ads on competitor ASINs, complementary products, and even your own top-selling ASINs (for cross-selling).
Consider a small auto campaign for each individual underperforming ASIN to discover new targets.
Bid Competitively: Start with decent bids; you might have a higher ACOS initially as you build sales history for these ASINs.
Optimize Product Listings: Ensure each individual child ASIN has high-quality images, compelling bullet points, A+ Content, and relevant keywords on its detail page. Competitive pricing and in-stock status are also crucial.
By giving these underperforming ASINs their own dedicated ad spend and detailed targeting, you're essentially building their individual sales history, which will eventually improve their organic and paid visibility.