It's clear you really take good care of your garden! Normally you just need to change the combination of plants in your greenhouse to have a chance to see a different special creature. However, since there are clearly more combinations of plants than there are creatures in the app, it may be that the same creature will continue to visit your greenhouse! If so, don't worry! Keep taking good care of your plants and you'll be able to change the combination in your greenhouse once more and welcome a new creature to come visit!