Take a look at the front of the burner, I've seen the baffle at the front of the burner rust out and fall off before.
You say you smell gas, so the ignitor, flame sensor and coils have to be working or no gas would come out to smell gas. If the baffle on the front end of the burner is gone, gas has a straight shot down and out burner tube, no turbulence to bounce gas back to the ignitor that sits to the side of the gas flow so gas doesn't hit hot surface of ignitor to light off then gas shuts off when flame sensor resets because of no ignition then repeats process.
Also check to make sure you don't have any kind of obstructions in the burner tube itself, like spider nest or other insects.
Here's what the burner should look like: