The mechanic is simple — ask people to post with your tag — and the failure rate is high. The reasons are consistent.
Failure one: no reason to participate
"Use our hashtag" is a request with nothing on the other side of it. Participation needs a reason: being featured, entering something, joining a group with a shared identity, or getting help with something.
Featuring is the cheapest and most reliable. A weekly slot where you repost the best entry costs nothing and creates a genuine incentive.
Failure two: an empty tag
Nobody joins something that looks abandoned. Before you ask anyone, put twenty of your own posts on the tag so an arriving visitor sees activity rather than a void.
Failure three: too much friction
Every additional requirement — follow, tag three friends, use two hashtags, post to Stories — cuts participation sharply. One tag, one action. Complexity is where campaigns die.
Rights, briefly
Posting with your tag is not a licence to use someone's content in advertising. Ask before you use anything commercially, credit clearly, and honour removal requests immediately. Getting this wrong is both a legal problem and the fastest way to kill a community.
Measure the right thing
Count contributors, not posts. Fifty posts from four superfans is a different outcome from fifty posts from forty people, and only the second one is a campaign.