If you have some suspicions about a blockage in your ventilation, then you might actually be able to handle the situation on your own. To help you identify and potentially remove the problem, here are some techniques you can use:
Check Vent Covers
If only a few rooms seem to be affected, then you want to take a look at the vent covers in each affected room. This will often be the easiest approach, since you will only need a screwdriver, a vacuum with a hose attachment, and maybe a flashlight.
You first want to run the vacuum over the exterior of the vent cover, removing any dust that might be clogging up the air duct. If that still doesn't fix the flow of air into that room, you should remove the vent cover and peek inside with the flashlight. If you can see a lot of dust, then cleaning it out with the hose attachment is a good idea.
Make sure that you tightly replace the vent cover when finished.
If you still think that you have a problem, and if that problem appears to be spread throughout your entire building, then you should go look at the filters on your heater or air conditioner. Naturally, you'll want to look at your heater filter if you are having air flow problems when the heater is on, and the air conditioner's when you are having problems with the AC.
Filters should be examined and cleaned once every few months, with increased frequency during periods of intense usage. If you find that your filters are getting dirty far too frequently, then you might need to consider replacing your permanent filter.
If you are dealing with disposable filters, then make sure that you are actually disposing of the filter and replacing it instead of just cleaning it off.
Find the Blockage
If you are still struggling with a blockage, then you can narrow down the problem using your building's HVAC systems. Simply turn on the heater or air conditioner and then close up your house as thoroughly as possible. You want each room to have a self-contained temperature in order to pinpoint the location of the blockage.
After waiting half an hour or so, you want to manually check the temperature in every room. If a room doesn't feel like it has been getting hot or cold air, then you should make a note and move on. Once you have compiled a list of affected rooms, then you can try to figure out where the blockage lies in your ventilation.
The blockage will often lie somewhere between two forks in your ventilation system, with all of the rooms after the blockage being affected. Even if you can't actually reach this blockage on your own, knowing where it is can go a long way towards solving the problem. If nothing else, it will give you some information to give to the professionals that come to clean your vents.