It is actually when the primordial black holes are too large, that we can't find them with lensing, we know the mass density of dark matter and if the black holes are large on average, then we would not see them with lensing because lensing events would be too rare. If the black holes are smaller, then there are a lot of lensing events and we can detect them. At somewhat larger scales one expects to see black holes due to interactions with stars.

That leaves a rather small window in between were black holes as dark matter works, but nobody has postulated a plausible mechanism to only generate black holes in a narrow mass range, so the idea can be made to work, but fell out of fashion because the wimp miracle looked theoretically more attractive.