One thing you can do on butts, Ken, is bump the temp up to 235. I've gone to this temp for ribs and butts, and it's working well. Butts, sometimes, can be really stubborn! Depending on how much internal fat and connective tissue there is, in that particular butt, will determine cook time - not us! You can even experience a "secondary stall," which I believe you did when it stopped climbing at 188 - been there, done that! Just gotta let it work its way through, and trust your smoker and thermometer.
Remember - meat is made by animals, and every one is different! No matter how much we want to control it, it sets its own agenda - despite our best plans! To me, that's what makes BBQ so fun, and challenging! You just can't control everything.
