Your venting, although clean and clear, has too many bends. The dryer doesn't have the velocity to push the air out. You must allow as straight a run as possible for your dryer to work properly. If needed, as mentioned above, you should install a helper fan, or booster fan.
Although a bend doesn't seem like much, a 90-degree bend to the simpler exhausting of a dryer is restrictive.
Calling an appliance repairman to resole this may frustrate your wallet.
If I were you, I would calculate moving the dryer to a new position with as little friction as possible since you must take into thought the gas line. Use PVC if possible for your exhaust.
Good luck to you, I know this can be frustrating.