After you select your filters in a custom campaign, click "Update Results." This will give you a list of recipients that fit the criteria you chose.
If that shows 0 recipients, it is likely that either none of your patients fit the criteria or some criteria conflict with each other. For instance, if you've selected two or more filters that include a timeline ("unscheduled patients who haven't been in for at least 6 months"), make sure the time ranges don't cancel each other out.
Another reason you may show 0 recipients is if you've selected codes with the wrong prefix (i.e. chosen the codes that begin with a "0" and not a "D"). When in doubt, always select both prefixes. For instance, when choosing the codes for a temporary crown, select both 02970 and D2970.