Dryer appears to run normally, except it takes 3X as long to dry a load as before. Tech replaced the control thermostat and checked the temperatures for different settings and said it was working properly. Tech called a second time because problem persisted (he didn't fix it the first time) and he found no problems, but it still takes 3X as long to dry a load and it is not very warm inside the drum as it was when it dried in a timely manner. The vent is not plugged. What is the problem?