Hi @timstephens
Could be a problem with the blend door actuator or the blend door sticking.
You didn't specify the year model of the vehicle but here are 2 videos that show how to replace the actuator, in a 2005 Tacoma that may help.
[https://www.youtube.com/watch?v=WMR1bfLfIrc|video 1] [https://www.youtube.com/watch?v=YZuVnbrpiRc|video 2]