The key to getting pork to fall apart is to cook it low and slow, typically about 8-10 hours. If you try to rush the process and cook on high or run out of liquid in with your roast, these are often the reason why it won’t pull apart.  Make sure there is a bit of moisture in with the roast (juice, broth, water etc.) and continue to cook at a low temperature until it reaches a point of being able to shred. Severely over cooking a roast and letting all moisture cook off will also leave you with a tough roast. Your roast's cooking time will vary depending on size.