Pork didn't get done in time
Based on your original question and the comments, it sounds like you simply did not make an accurate assessment of the temperature of your product and your cooker.  Low and slow is an excellent technique to make pork shoulder tender.  Your goal should be an internal temperature of about 195 F ((90C).  Estimates are that you should allow 1 to 1.5 hours per pound, but this can vary depending on the heat source and the fat and bone content of your pork.  It is best to measure the internal temperature.  As pointed out above, two things potentially "went wrong".  Either your cooker did not maintain the temperature you thought it did, and/or you pulled the pork out before it reached the desired temperature.  For best results in the future, use accurate thermometers and measure both.