AC not blowing Cold in Part of the Upstairs

I have central air with one outside unit that cools the downstairs, and one that cools the upstairs. Downstairs is fine. Recently upstairs I noticed a difference in the air temperature coming out of the vents in just some rooms upstairs (not all) - about half the upstairs is cooling well, the other half (including the room the thermostat is in) is blowing cool, but not cold air. I have not taken the temperature of the air from each vent directly, however I can tell you that just placing your hand on the vent you can easily tell there is a difference in temperature between them. Since the same outside unit and blower control the entire upstairs area I don't understand why the air would be colder in part of the upstairs area and not others - I would think it would all either be cold or it wouldn't be. Anyone have any ideas of what to look for? I have an Air Conditioning tech scheduled to come out this week.