Yea, the whole navigating layout and servo-hunt trophy could've been better.
A few times while playing the Blood Angels campaign, there will be multiple missions available to you. Always "inspect" all, and always do the optional missions first, if any. The dialogue might give you clues to this. For instance at some points, the Ordo Inquisitor wants you to make a detour.
And near the end, you need to almost go out of your way to find the emperor's seal and play "Relic", before doing the last story mission. If I were to guess, this is the one you missed. A loaded game, might prove.. bountiful, if you have one available.
As you probably noticed, new sections of the hulk will sometimes be made available, depending on your choices at crossroad. Save/Load can be helpful for exploring/experimenting. Good luck.