Junk,
I wasn't joking about checking out the buccal cavity, upper throat. Large items like marbles, or large gravel, aquarium equipment..etc can get lodged on the way down. You mant want to ask someone to help you do this. You'll have to dampen a towel with aquarium water and do this over top of the towel, just incase the fish squirms out of your hands. You'll want to make sure you're holding the fish securely but without too much pressure. Use your finger nail to gently pull down the lower jaw to open the mouth. You should try to do this as quickly as possible (no more than 30 secs for the whole process) as you want to keep stress to a minimum. If the marble is lodged, surgical forcepts can be used to gently remove it. This is quite serious, and perhaps you may want to do further reading on the internet on how to safely handle your fish, and safely open its mouth. You can probably find loads of articles by searching for how to strip an african mouthbrooder of her eggs. And please, in the future, no more marbles.