That's a bit hard to explain, so bear with me..
Chris has no theory of mind, he doesn't understand the larger scheme that the trolls are trying to pull, and he clearly doesn't understand how (essentially) the same trick keeps working on him so many times despite all the measures he takes.
This is because fundamentally, Chris doesn't understand how much his autism contributes to his life problems, versus the contribution by his isolation and lack of actual life experience. He thinks that we have all found the perfect algorithm that helps you find life mates, and we're all hiding it from him. This also manifested in his then-constant pleas for "Date Ed" to become a thing.
So whenever he is trolled, let's say by Ivy, he learns to not trust a trollsona even if they talked to his parents on the phone beforehand. But that doesn't cover OKCupid for example (Catherine's initial 'in' was from there, IIRC), because he only learned how to prevent this particular permutation or this infiltration technique. So when the next trollsona comes along, Chris won't blink an eye, as long as she comes with a new "In", like Jessica did (Basically befriending him on FB for a long ass time, get him to trust, and catch him while he's desperate).