There are a few things to double check if your ads are no longer serving in your DSP or social network.
Every DSP has different parameters around what they consider an "active" device for serving, so it is recommended to republish your audience throughout your campaign to ensure your audience does not expire in the DSP and to add fresh devices to your audience. More on republishing options here.
Check that the audience is valid (not out-of-date). If the audience is out-of-date, clone it with new dates within the 12-month lookback window and re-publish the audience.
If your audience is on a publishing schedule, check the date of the latest publish, and re-initiate the schedule if needed to keep adding new devices to your audience.
If you have double checked these items and the audience is in your DSP seat, but ads are still not serving, you will likely need to review your line item campaign setup including bid pricing, etc.