nothing to worry about. she should stall around 165* give or take a few degrees and hang there for some time.
a couple of things to think about:
1) where are you measuring you wsm temp? its not critical, but if you are taking a dome temp opposite the top vent, assume the 250* is low. That's a perfectly fine brisket temp, and basically what I run, but don't be surprised if things go slightly faster than anticipated. But remain patient, its done when its done.
I just picked up an ET-73 (I needed the alarm so I can get some sleep at comps) but never used it. Now that I think about the set up, I'm going to assume you have the cook probe on the grate, so you can disregard most of what I stated about.
2) probe placement in the flat can make temps deceiving. its not really thick so if you aren't in the exact middleof the thickest part you aren't reading the coolest temp in the brisket. Not a problem either, as temps are only a guideline.
good luck.