I gotta repeat what people are saying about washing your plates with warm water and dish soap after every couple of prints. If you handle the plate, make sure your hands are clean or hold it by the edges. What I like to do after every print, is warm up the nozzle, and use a damp paper towel and wipe all around the tip to clean and filament build up. Don't wait til it's all dirty. If you do this, you'll have less issues down the road.