Some accounts on Slack are Guests, either Multi-Channel Guests or Single-Channel Guests. By default Donut will not include these users when it makes pairings, even if they're in the channel.
If you'd like Donut to include guest users in pairings, sign into donut.ai and select the channel you'd like to edit. Click the gear icon of the channel you wish to edit.
Under the "Basics" settings tab (near the bottom) you'll flip the switch for guest users.
Don't forget to hit "save" to update your settings!