This seems to go on month on month and the developers don't seem to understand why lots of "casual" players (casual means players who do VE to N V1 in groups) will group up and do Ambuscade one month and not others. They will even make mechanics to persuade them to group up (like two different kinds of weapons skills required), but ....they still don't group up for some reason.
It's very simple, they don't because you make a tank compulsory on all difficulties and it's very difficult to get a tank. The few tanks that do want to do Ambuscade do not want to do VE to N and so barely anyone groups up for Ambuscade on those months.
Every MMORPG that requires a tank has a massive bottleneck, because tanks are always limited resource and always picky. This is why you wait hours for group finders to finish and it's why few bother making groups on months that require a tank.
If you want people to group up and start doing more Ambuscade you need to lighten up on the requirement for a tank on VE to E, the tank population is very very small and very selective. It's near impossible to make random groups for this level content, it's easier to just solo it with a trust tank than make a group. Which means more solo players doing lesser difficulties and longer wait times for everyone else.
When an Ambuscade allows players to sleep adds on VE to N then you can build a group with more DD jobs and skip the tank, when an Ambuscade lowers the attack or removes hundred fists from those monsters on VE to N then more players will create groups.
If you want more grouping of casual players then you need to stop making people reliant on tanks for lower difficulty modes, that you need a tank on VE and E is ridiculous. All you do is ensure nobody does it those months in a group.