Initial studies were done with non-vaccinated risk groups and it showed a good efficiency. Since then most people have already been vaccinated and the benefit for them is probably lesser. The post-marketing studies showed that it doesn't help people under 50 y.o.
The other issue is with covid rebound when using Paxlovid. In the studies rebound was something like 2-3%, in practice it happens much more often which can also indicate that the study population that was benefitting from Paxlovid is different from the one that is taking it now.