If you have a gas dryer and can see the flame that heats the dryer, then watch the flame. When it clicks on the flame should point straight down the vent that goes up and into the drum. This is how it should work when everything is clean.
If the flame burns like a candle, like the flame goes straight up an not follow the vent, then there is not proper air in the dryer. This causes the dryer to heat up to quickly and cuts the heating cycle by reaching the thermostats cut-off temperature too quickly.
I'm not sure how you can tell on an electric dryer.
If you are any bit handy look up on the internet how to take the front off your dryer. Then unplug the dryer and slide it out enough to get behind it and take the vent hose off. Slide the dryer out and take the front off while it is unplugged and vacuum the inside out really good, paying attention to the motor and getting it clean. Get a brush like Niall C suggests and clean out the vent in the wall. Hopefully your vent is less than the maximum length 25 ft (in the wall), taking footage off for 45's and 90's. Too long a vent will cause your problem too.
If you have a plastic or vinyl hose that attaches between the wall and your dryer then replace it with Mylar, foil or aluminum. When you put the dryer back make sure the hose is not crinkled or crimped anyplace. A crimped hose will causes it to act like a dirty vent.
I was a Maytag repairman many years ago, luckily it was commercial only.