There are a couple of different things that could be causing the no-bake cheesecake to be soft and not set. It might need more chilling time. I find that it definitely needs at least 4 hours to set. If your dessert bowl is larger, then it might need more time. Second, the amount of agar-agar might not be sufficient. Try increasing the quantity a little more next time. Even if you have a slightly unset layer, the dessert would still taste great. So go ahead and enjoy it.