Explanation
A campaign delivering well below its eligible impressions is being outbid, running out of budget, or being held back by creative the auction does not favour. Those three causes need completely different responses, which is why the metric is worth separating from raw delivery numbers.
On short-form placements specifically, creative quality feeds back into eligibility more strongly than on static formats, because the platforms are pricing partly on predicted engagement. A better hook can raise impression share without a bid change.
The common mistake
Raising bids to fix a low impression share. If the cause is creative rather than price, the higher bid buys the same delivery at a worse cost.