I have found that telling when a brisket is done is not a temperature thing. Use temperatures as a guide only, but they are done when they feel like they are done. The old 'when a probe goes in like into butter" test.
Also, every brisket has a different time to be done -- it all depends on the structure of the fat and collagen of each brisket.
You might have had a brisket that was just too lean also.
Flats are one of the hardest things to consistently smoke successfully.
There are a lot of people with a lot more knowledge than I who will chime in, but thought I would add my 2 cents worth anyway.
Jim - Another transplanted Texan
Former KCBS CBJ
Large and Medium Big Green Eggs , Black 18.5" WSM, Blue Weber Performer - Stainless, Green Weber OTG Kettle , Brinkmann SnP Pro, and a Stainless UDS. One retired Portable Kitchen grill.
Red Thermapen, Maverick ET-732, EdgePro Apex Sharpener.
Avatar is the original 1951 Weber Kettle