Once the last date for booking has passed, there's unfortunately no guarantee you'll be able to make a booking. Some venues are happy to keep taking bookings where they can, but they are more likely to become full once the book-by date has passed.
You're welcome to ask the venue nicely if they can still fit you in, and they'll hopefully be able to help. If not, we're really sorry to say your voucher will be considered forfeit.
To avoid disappointment in future, we'd always recommend booking as early as you possibly can.