What could be wrong with our oven?

Hmm, cleaning may help. There may also be a problem with the coils(or whatever they're called lol), they could be damaged in some areas. Also you may want get an oven thermometer and see if you're oven is actually heating up to the temperature you are putting it on. Maybe try lowering the rack you cook on, but that's not necessarily a good idea unless you watch it closely cause you could over cook or burn the food. You may also want to rotate the dish at different times during the cooking, maybe that will help it cook more evenly. If none of that works you may want to get a hold of your maintenance at your apartment complex and have then come check it out.