Kinda hard to say but it sounds like you may have pulled them a bit too soon. I tend to mix up a baste and mop them every hour or so after the 4 hour mark. I don't know if the baste helps with tenderness but it does seem to help with moisture content. Don't give up though. It's brisket on a stick and needs to be cooked the same way...a long, long time till melted butter tenderness. I'd guess that pretty much any cut, if cooked long enough, can be made tender so I'd rule out the cut first and take a stab at cooking a bit longer. Outside of that, from a visual perspective, it looks like you nailed them.They were not as tender as I thought they should be. That could be from the cut or maybe letting it ride longer?
Robert