1) The feeding size is more than dimension when it enters into mill. 

2) The straight pipe is not under right adjustment. 

3) The moisture of raw material is big, which cause some attach on roller and ring.  

4) The grinding ring wears too much to keep circle. 

5) The feeding can’t be supplied enough. 

6) The raw material with higher powder content is so light that mill vibrates violently. Just reducing blowing will be useful.