I'm not an RF expert, here are some things to try (feel free to disagree if you find any of my suggestions ridiculous)...
1). Has the whole wireless
system been effectively RF coordinated? If so can you give us some more information - number and make/model units you're operating, frequency ranges,
etc. and what process was used to select frequencies, if you used software or let the units find their own frequencies,
etc. And frequency of the actor's pack in question.
2). Do you have a
portable RF scanner? Might be good to do a sweep of the area inside your
venue with all transmitters off and in different locations to identify potential conflicts. Some coordination softwares will let you import these scans as files to help coordinate the whole
system around the used spectrum in and near your particular
venue.
3). Don't rule out a loose antenna on the pack. Actors are tough on the packs, I've seen them pull the pack out of their pockets by the antenna (yikes). On some packs a sketchy antenna is easy to repair if you're handy with a soldering
iron - have to be careful not to change the length of the antenna tho. Should be relatively easy to diagnose too. Just do a antenna wiggle test with the
channel on and then twist the antenna slightly (being very gentle and careful of course). You will find out in a hurry if your pack's antenna might be the issue.
4). There are some different antennas you could try on the receiver that may help the problem if it still persists, maybe a
paddle antenna might help. RF
Venue has some interesting options.