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Al is right if they are tough they are not done. I use the bend test. Works every time for me. Basically after the ribs have been in the smoker 3 or 4 hours I use a long 16" tongs and slide it halfway under the rack of ribs and pick it up. If it bends and the meat just begins to crack that is perfect for me. If they actually break in half they are over cooked.
And as Al said we need to know more about the method you are using now in order to help you correct your problem.