The problem with possums is that they don’t seem to be deterred for any length of time by anything.
Studies by Deakin University where they used a controlled test looking to see what would stop possums eating gardens. They tested 14 products some natural some commercially available. They acted on 2 senses, the sense of taste and those which act on the sense of smell.
Using 7 of each, the bottom line is that none of the products were totally effective. After a while they just ate the test sample anyway.
The same thing goes for stopping them getting into your roof. If some changes from what they’re used to, they will just watch what is happening and if they see that nothing is going to harm them they will resume their normal routine. The only way to stop them getting in is to physically block the holes after they have gone out.