Try this, remove the vent duct from where it attaches to the dryer and clean out the obstruction of lint etc. that you find there. Chances are this might fix it. If this doesn't do it, then turn on the dryer while the hose is off and see if you get a nice strong blow. If you do, then you might need to replace your dryer vent hose/duct. These can get a buildup of crud. Avoid sharp turns with the hose/duct and maybe check the flapper outside to make sure it isn't stuck or clogged.