The bulkheads in PE are always blocked, on all VS aircraft. You can never select them yourself. It can only be done at the Airport. Thats because most bulkhead seats are bassinet positions.
Check in and CH staff can arrange for them to be unblocked and for you to be seated in them.
Personally, I check in late for VS flights, because they do all of their upgrades manually, meaning check in time, fare class, status and number of people in the travelling party - all come into play. Single passenger showing up last is usually an easy way to get upgraded. But that's whole separate threads worth of conjecture!