Here's a little quote from user 'jimc'

i switch plastics alot and i do have to clean out my hot end and nozzle often. if you have never cleaned it then im sure your due. before you do anything, pull the nozzle and brass tube off the machine and soak it in a little paint stripper. the methylene chloride will dissolve, pla, pet and abs. basically everything except nylon. let it sit a few hours then rinse, blow out, run a pipe cleaner through the tube, etc. Use zip-strip from Home Depot.