What can cause a blockage in a sliva duct?

Blocked Saliva Duct. A blocked saliva duct is caused by a mucus plug or a stone. Either occurs slowly, over time, and may lead to sudden blocking or plugging of the saliva duct. This will cause sudden swelling of the salivary gland. The two largest salivary glands are located in front of each ear, the parotid salivary gland. Drink plenty of fluids, massage the affected salivary gland, and suck on sour candy.
Salivary blockage. Usually it's due to dehydration or someone on meds like "water-pills". Drinking a lot of water and using sour candy or chewing gum to promote salivary flow is helpful. If it continues the salivary ducts and glands need to be thoroughly examined by an ent.
Bacteria. There is a condition known as sialandentitis caused by a bacteria in your salivary gland which can cause stone formation and thus causing blockage.