>>>Cooked at 225 for 6 hours<<<
as others have said, that's the problem. try cooking closer to 240 or 250 for many more hours - it might even be twice that time, but it is worth it.
wal-mart has a very decent digital probe thermomenter called the accurite -it's a little under 15$ here, but is extremely accurate. i run it though a small potato and keep it at grate level (not touching the grate) throughout almost all of the smoke, using it to keep grate temps at the level described above. when the meat starts to get very dark on the outside and it's been "long enough to start thinking it might be close to being done," i check the internal temperature now and then at the thickest part away from fat or bone. the probe should pretty much fall right through and look for 192 MINIMUM internal temperature. 195-200 and even 205 isn't out of line. over 205 and you start to really lose all that good moisture and tenderness that you ahve been nurturing all day long. when it is done, wrap in foil, then a couple of towels, then put it in a cooler to rest at leat a half hour, but it can rest as long as the internal temp is above 140. when you unfoil it, it will be piping hot and will most likely fall apart. save the juice! then all you have to do is take a picture for us, finish the pulling, take another picture for us and serve it up (take another picture here, too).
any thermometer should be checked to make sure it is accurate. make an ice and water slurry and check the temp - it should be very near 32 degrees, only a half a degree above at most. then boil some water and check the temp there AFTER checking on the internet to find out what the boiling point of water is AT YOUR ELEVATION. mine is 207.8 degrees where i live and myu wal-mart themometer is dead-on.
the ebst thing about BBQ is that even your mistakes taste better than food that most people pay for. try again, and enjoy! when you get it right, it is SO worth it. there is something about going to turn a shoulder over and having the bone come right out or a big piece slough off that just feels right!