A passive antagonist produces a sag every time, because pressure has to come from somewhere and a plot cannot generate it on its own. The fix is not making the villain nastier; it is giving the opposition a want of their own that cannot coexist with the protagonist's.
Write the antagonist's logline as if the film were theirs. If it is a worse film than yours, fine. If it makes no sense — if they have no goal, no deadline, no cost — you have an obstacle rather than an antagonist, and obstacles do not escalate by themselves.
The strongest oppositions are the protagonist's own argument taken further. The mentor who believes the same thing without the doubt, the sibling who made the same choice and did not flinch. That is why they are frightening: not because they are alien, but because they are adjacent.
Give them a win. An antagonist who never succeeds is scenery. Let them take something in the first half that the protagonist cannot get back, and the second half writes itself.