I don't know, perhaps he just didn't know enough about it at that time. I think that, when Comey revealed that he had a friend call for an investigation into the collusion with the Russians thing, that might have been the catalyst for the firing. This is just speculation on my part.I'm still curious to find out who talked trump out of firing Comey right after inauguration day. so many people wanted to see him go. at least one person who had trumps ear managed to change his mind.
probably not sessions?
I think that, after Comey did his dog and pony show, making an excellent case for indicting Hillary Clinton, and then letting her off the hook, should have prompted Trump fire him on the afternoon of January 20, 2017.