Marco, looks like you covered a venting problem and with the fact that you changed all the thermostats, leads me to believe that your primary and boosting coils are not functioning as they should. This are located on the gas valve. A sure way to tell is by observation. The ignitor will glow, you will hear a click and the flame should start. If it glows, clicks and does not start you can be assured it's probably your coils. Pretty inexpensive to repair. The part numbers are 5303307292 for the secondary coil and 5303307292 for your booster. Make sure to change both.
If your dryer is not drying like it once did or if you just have not taken the time to give yourself a clean dryer in a long time then you need to do that. At least once a year the dryer cleaning should take place. Lint build up in the dryer duct is one of the main causes for your dryer to fail.
Here is the rest of this tip about Dryer Maintenance
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