A Model Number may help.

Some units have two heating coils, they turn one on for low heat and both on for high heat. If your unit is this type perhaps one of the coils is burned out.

Some times a heating coil will deform and touch the case re: a grounded element. This also can cause low heat.

Could be you just have a dirty/clogged vent system.
Undo the dryer from the exhaust
Run a load
If the load dries correctly then you have a vent problem.
Clean/check the vent system. Make sure the outside louvers open fully.

If you do not want lint in the house.
Take a pair of pantyhose, put one leg into the other and attach this to the dryer exhaust.
Leave enough room behind the dryer so it can blow out like a windsock.
Run a load.

Both the above will allow you to check the air flow and temp.
Also the above is not recommended for a gas dryer due to possible carbon monoxide problems.

Here is a good link with troubleshooting help
http://www.applianceaid.com/dryers.html