[cypher]

Which species of hamster do you have?
Russian hams have a scent gland on the abdomen which can often get a bit blocked & needs to be cleaned by gently bathing with damp cotton wool, if it isn't cleaned it can get infected which would then need to be treated by a vet.
With any other species I would wonder if your ham had been sleeping in a wet nest?
They are usually very good at grooming & keeping themselves clean so I would give your ham a good health check to make sure nothing is wrong there & nothing is preventing him from grooming properly.
If you don't have one already a sand bath with chinchilla sand, not dust, is very good for them, as well as digging in it they will often roll around in it which helps to keep the coat clean & in good condition.