Furnace ignites only two burners out of four. Propane

Hello I have a american standard furnace approximately 30 years old. I have replaced the igniter, flame sensor and gas valve was replaced two years ago. The furnace tries to light on first try, it goes through all the motions and when it calls for propane it lights first two burners out of four and sometimes the last two will light after a few seconds. The furnace sometimes cycles and lights on it's own, but usually daily I have to switch it off to reset it in order for it to ignite. Any help would be useful. Thanks in advance. Robb