The racks could be bowing and causing this problem if they are worn or damaged from previous heavy loads. If the upper oven racks are the same size as the lower oven racks, you can see if the rack is causing the problem by trying the upper rack in the lower oven. If the upper rack works properly in the lower oven then the rack that was being used in the lower oven is apparently bad. You can order a new one from the Sears PartsDirect
If the same problem happens with the upper oven rack when used in the lower oven, you could have a problem with the sides of the lower oven cavity being bowed or distorted. This type of problem would need to be examined and repaired by a service technician. Here is a link for the Sears Service website: Sears Home Services
If you need more help with this problem, reply with additional details and the full model number of your Kenmore 790 range.