They do, but there are two problems. One is that that still leaves a pretty big range for possibilities for where the planet could be. The other is that we don'

They do, but there are two problems. One is that that still leaves a pretty big range for possibilities for where the planet could be. The other is that we don't know where along the orbit the planet is now. Even if we knew the exact orbit of the planet simply not knowing where it was along that orbit means we'd have to scan a lot of the sky to spot it. Having a bit more ambiguity about the orbital parameters means we will have to scan much, much more of the sky to spot it.