What Causes Thick Salivation And Phlegm Caused After A Brain Stroke?

Am stroke survivor. Stroked both sides of brain and in brain stem June 2014. Had extensive pt including swallowing therapy with barium swallow tests and endoscopy; everything normal. Feeding tube no longer needed. However, am still troubled by thickened saliva and phlegm. Primary care physician and neurologist have no idea why the mucus remains thick--sometimes from head and from esophagus when swallowing hard (not always though). Any ideas why and what I can do to make this better? Thank you, Patty L.