all the time.
a/c heator wont wont blow air of vents on 1999 Ford F-250 Super Duty
Naturally - also look for disconnected or damaged vacuum hoses.
If problem is intermittent then could be a vacuum servo that controls blend door operation, if remove glove box then look at servo linkages and see if bleed off when it does it. Sometimes the controller in dash will have vacuum problems as well.