i've never had a problem with my blended sours carbing. youngest part is around 6 months. oldest, about 2 years. they'll take longer to carb, and won't ever get highly carbonated, but imo, they come out perfect with the usual amount of priming sugar you'd use on a typical non-sour ale. but it depends what you want. if you're looking for sparkling, foaming, heady carbonation, i'd say just prime more and wait longer. i don't think adding additional yeast is really necessary.