Hi TommyD!
How are you prepared for Irma? I hope you are not in its path but if you are please be careful!
I've never used an electric smoker but I am guessing its cooking process must be similar to traditional wood/charcoal smokers.
My first observation is that 3.5 lbs is a bit small for a brisket. Most that I have cooked are at least 10 lbs - I have found that the sweet spot is between 13 and 15 lbs - these yield about 8 to 9 lbs of finished product. I understand that this is a lot of meat for one or two people but with briskets I beleive that size really does matter! You just need to invite lots of friends to come help you eat it! This is why you may not be getting the bark you are looking for since it is not in the smoker long enough to develop a good thick bark. The recommended cooking time for brisket is 1.5 hours for every pound of raw meat, so even a 3.5 lb cut should cook for at least 5 hours at 250 - with the internal temperature being around 195 degrees when done. The bark is directly related to the amount of time the meat is sumected to the heat AND smoke.
For me, a brisket is as much about having my buddies drop in over the cooking period to help tend the coals, drink some good bourbon, smoke some cigars and have a good old fashion GLUE session (GLUE - God, Life, the Universe and Everything).
Hope this was helpful! Let me know how your next one comes out.
Peace!