Dryer venting problem
Add both a secondary lint trap and a dryer duct booster fan. Fantech and Tjernlund are a couple brands to help you start your search. 

Also, replacing any easily accessible flexible duct with rigid ducts can help prevent lint from building up. 

Finally, check that the exhaust louver/vent cap outside isn't filled with lint too. Sometimes contractors use dual use intake/exhaust vents and forget to take the intake screen/mesh/filter out that is used to keep bugs from flying inside when used as an intake.