If you've already read our articles about audiences and remarketing you'll know that adding audience data from either first party sources or, making use of an ad partner like Google or Facebook to leverage their data can seriously improve campaign performance.
It looks like you've started making use of this but, in order to protect privacy most ad platforms require an audience size to be at least 1000 users or more, this means that we're not being too invasive or isolating particular users. Rather we're identifying a series of traits or behaviours at a top level (which we hope is far more useful and far less creepy!)
To resolve this issue there's a few things we can try:
Check the tracking - Is this audience set up correctly? Is it getting properly populated or is it missing any data? If we can push more users into it then we'll have enough data to qualify for it being useful in our campaigns.
Widen the parameters - if everything is tracking correctly but you still don't have enough users then you might consider widening the parameters, for example if your audience is newsletter subscribers to a particular product, you might try expanding the audience to include subscribers for a few different but similar products to increase the audience size whilst remaining relevant.
Use advertising to fill up the audience! - Another great option might be using your campaigns to help you add more to the audience, for example if you're looking to remarket to visitors of your offer page but you haven't had enough visitors, then why not launch a new campaign specifically directing potential customers to the offers page!