What Causes Constant Feeling Of Gagging In Throat?

Hi I have difficulty swallowing. There feels like there is something in my throat. I am constantly clearing it.
I have had numerous tests done but they can't seem to find the problem. They checked my esophagus,
stomach, watched me swallowing through a machine like an x-ray machine but couldn't find anything.
Do you have any idea what it could be.