Outlined in the following Reddit post, Instagram now sets hard caps on your account's reach:
https://www.reddit.com/r/Instagram/comments/14jt40q/cap_on_daily_reach
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An obvious corollary of this is that if your Ads reach the quota, the organic reach of your new posts will be quashed!
// this is especially true near the end of the day (California time) where you have already saturated