Combines rear feed chain is not straight

could be that chain is off one kog to fix that we use a 3inch piece of a broom handl just slip it under the chain and roll it over the sproket and it will jump one kog foward the broom stick can fall and just let it go through the machine. the only other thing it could be is the sprokets could be out of time they should have a little notch in the spoket tooth youline up to time them the same. if this is wrong you wlll have to pull out the shaft and retime the sprokets

Yes your sprockets are out of time use the set screw to aline if there is no mark. Do not use bolt holes in the sprockets or you will have only a 50% chance of being right and have to take out again. If you have been running wrong the chain may have moved and you will need to loosen all slats and rotate chain around then retighten them.