That's a waste if no one is home.I would wait till evening and open all the windows and get some air flowing to lower temps.

That generally increases air flow to the other vents (so hopefully more upstairs). 

If the blower wheel is plugged up full of hair, get an HVAC tech out to clean it.Go around the house to all of the air vents, and make sure the flaps in them are open (use the little handle/lever/knob that sticks out of the grille), and that they don't have a rug or furniture or something blocking them off.Sometimes people will turn the thermostat down and then close the vents to try to regulate the temperature - if this goes on long enough, the evaporator (A-coil, inside coil) can get ice on it, which really blocks the air flow. 

We recommend vents be properly installed to allow air flow to move inside the crawl space."

The room is not completely air tight, there are gaps near the ceiling joists that would allow air flow.

You can't block air flow or the cool air will not reach everything.

Could be from improper insulation, blocked air flow, and other age issues.

If the air filter is dirty it can restrict airflow causing it to "run slow".

How important the A/C is will depend on where your rental property is located and how hot the temps get there and how many months the hot weather lasts.I would just include replacing the filters as a bonus to the tenant.Filters say they last 3 months but really it is not true.This is especially if you have a smoker or a tenant with pets.What happens is air filters should be replaced if the cheap ones about every month to 2 months max.Expensive ones you can go up to 2 months.If you wait 3 as more dirt is attached to the filter it restricts air flow and won't pull in as much dirt.When this happens less air will flow through the vents and the tenant will turn the unit down more and the resistance will make the motor work harder and wear out faster.I was having this happen in my house.A/C unit is 9 years old.When it gets in the high 90's here the A/C unit was getting in the 80's.Has an A/C guy come out.I had the old analog mercury switch A/C thermostat which is off by as much as 5 to 6 degrees sometimes the guy told me.I use the Filtrete Ultra Allergen because we have 5 cats and was replacing filter every 3 months (cost about 17 bucks a filter).The A/C guy explained about air flow and that I need to replace filter every month.The high end filter will catch more particles he said and when you keep them replaced they are fine every month.If you wait the 3 months it will reduce the air flow to much with the dirt it catches.So I replaced the filter and he also checked my free on level which was fine but my coils needed cleaning.He just basically took a hose and cleaned it out after turning power off and before the 100 dollar bill was not sticking to the coil halfway to the bottom.After cleaning the bill was sticking to the coil everywhere.He said it's important that when the heat comes out the coil can cool it properly before it goes back in the house.I bought the electronic thermostat myself for 25 bucks and took out the mercury one.Now with it being 95 outside it is holding at 73 degrees just fine.So I would replace filter every 2 months at the most to preserve the motor and help air flow.On some properties the builder installed too small of duct lines for the air flow and run to the vents anyways so any little air flow restriction will make the unit hard to keep cool in the hot weather.Probably too much info but thought I would share.Hope it helps.

That flexible ducting has many bends in it which can decrease air flow and create more areas for lint to build up.