Everyone who has ever adopted a cat and brought them to their home knows that gaining a cat's trust is something that takes time. Cats are creatures who need space, respect, love (and lots of treats) to warm up to their new people. And sure, sometimes, it takes a few hours of this, and that's great, but other times, it can take years. And the latter is certainly true for feral cats.
You can never really know what happened in the past to a kitty that you rescued from the world, but chances are that if a cat has been living outside for a time, its trust in humans is on the low side. That doesn't mean that the kitty doesn't deserve a home and love. It just means that you might have to work a little longer to have that love given back to you. But it's worth it. Every second is worth it. Because when cats love, they love hard, and bring on the receiving end of that love is ameowzing.