Defrost cools down

We have a 2014 579 daycab with a MX13. The vents, floor and defrosters blow air and filters and cores are clean. The vents, side window vents and floor all blow hot air but when defrost is turned on the air slowly cools down until there is no hot air blowing.Turn knob back to different setting and heat comes back on. I do think the side window vents put more air out than they use to. Is there a blend door that might be stuck or another problem? Thanks for any info.