First; welcome to the forum.
You are correct that you could should have been finished as planned if you were at 225°.
Many variables can effect the outcome of your cook;.....
The size of the amount of charcoal used in the cooker in relation to the size of your meat. A small heat source has to work harder to keep the temperature up inside the cooker.
Dropping outdoor temperatures also cool the cooker as well.
There are differences in the same cuts of meat themselves which affect the cooking time.
Suggest try to raise the temperature and even foiling the meat at 160° to help get it through the stalling temperature.
Humphrey's DownEast Beast W/BBQ Guru