1. Your account has been shadow banned
Shadow ban limits your reach to only those that are following you. Accounts that are not following you won't be able to find your posts on the hashtags. To fix this issue, delete previous hashtags and take a break from hashtags for some days or even weeks.
2. You are using banned hashtags
Instagram bans hashtags that contains spammy content. To avoid this, check your hashtags before using them.
3. Your hashtags are too broad
Assuming your engagement rate is really low, let's say like 3% and you are using hashtags with 2M or more post density, what happens is the hashtags swallows your post in few minutes. You do not have the engagement to rank on those hashtags.
Use hashtags with small post density that you can rank on. Use hashtags that more specific and searchable. Use niche hashtags to attract users that are interested in what you do.
4. Your hashtags are irrelevant to your posts
Your hashtags may not always be a hit! I get low impressions from hashtags too. My last post got only 3 impressions from hashtags. I know what I did wrong and I will fix it!
Does that stop me from engaging and dishing out great content? Hell No!!! Hashtag or no hashtag, ensure your engagement and content strategy is solid!