PAX are only forced to book JetPLUS fares when they are running very low on available seats, around 3 or 4 left I believe. Anything before that is available as a X/N/Y fare which is your normal non-changeable bog standard ticket. So, unless you have 14 booked on a flight JetPLUS fares are always going to be available.
nav3, to book anything above 9 seats you need to start trying to book kiddy fares as well.