Question about a potential heater issue

[Steve S.]

I had a tenant call me and say one room upstairs is not warming but the master and other bedroom upstairs and the downstairs area is warm. 

I had them confirm the last tenant didn't shut the vent and they confirmed cold air in their opinion was blowing out the vent. 

The unit has 2 heater units, one for upstairs and one for downstairs. 

I'm perplexed as to how all bedrooms and hallways but this one bedroom upstairs could be hearing appropriately. 

I'm about to call my HVAC guy but does anyone have some obvious suggestions I may have missed?  They have company coming over in 4 days and I'd like to make sure they are comfortable.