Admittedly, I've not done an exhaustive survey of flights and available seats, but on all the flights that I've tried to book on A319s so far, seats 5C and 5D are always blocked.
Does anyone have insight into what's up with this? I'd ask if anyone might have an opinion on the matter, but the answer to that is obvious on FT
They might be reserved for the day of departure assignment for people with disabilities since to be seated in stretch you will need to pay. Midwest used to block the first row after they're signature seats for 'bulkhead' seating.