Poor air flow: If you have a conventional vented tumble dryer (air vent out through the wall) then it is not uncommon for the air vent to become blocked with frequent use. The causes of this include:

- the vent on the exterior wall becomes blocked.
how do i fix it: make sure the rear grill or louvre has not become clogged with lint. 
- the venting hose can get kinked behind the machine.
how do i fix it: ensure the machine is not pushed too far back against the vent hose.
- filters can become blocked with lint.
how do i fix it: remove surplus lint from filters as per machine.

Condenser dryers have no external vent but the condensing unit can still become blocked with lint. Consult your manufacturer's handbook to locate the condensing unit (somewhere along the bottom of your machine). Remove the cover plate and pull out the condensing unit. Place in a sink/bath and carefully flush the built-up lint away with water. Put unit back in machine and replace cover plate.