I've been tracking a similar pattern with a test account I set up recently. Warm-up phase was roughly a week of genuine engagement-likes, comments, follows in my niche (B2B marketing automation). First two carousels got literally zero impressions. Reels? A couple hundred each. That told me the account wasn't shadowbanned globally, but something specific to static multi-image posts was broken.
After digging through the API response logs (yes, I scripted that), the key insight was that Instagram's content distribution engine applies a completely separate engagement threshold for carousels vs reels. Reels get an initial push because the platform prioritises video; carousels rely almost entirely on the first slide's hook to earn that first 50-100 impressions. If the algo's internal scoring for that opening frame is below a hidden bar, it doesn't even surface the post to a test audience. Your reels performing fine confirms the account isn't flagged, but your carousel slides might be too generic for Instagram's early filtering.
I'd suggest a two-pronged fix: extend your warm-up to at least 14 days with real, niche-specific interactions, and test a carousel where the first slide is deliberately provocative or contains a bold data point. Also check your account's content delivery status via the professional dashboard-sometimes the 'healthy' label is misleading for static formats. If the zero-impression issue persists after that, it's likely a silent rate-limit on new accounts posting carousels too frequently. Space them out to one every three days and see if the numbers shift.
