Most likely, corrosion/dirt got further on the contacts. A flash could be discharged by firing it but I wouldn't really suggest to open up a flash if you are not familiar with this kind of electronics. Flash capacitors can really held some power on them and it could be dangerous. I would say hand it over to someone who is familiar with electronics or trash bin. Try pulling the contacts a bit further against batteries. Probably cleaning process flattened them and they are not having a full contact on battery.