I've had this issue with my D6. When it starts I quickly pull the nylon tube up out of the printer head, grab the filament and push it down into the hotend like if you were feeding it into the hotend for a filament change. This usually solves the problem. I don't know why it does this. I don't notice any filament grinding or anything like that. Clean the crap out of your hotend aswell. I had upgrade to a all metal hotend but I went back to the stock hotend as it seemed to have less issues.