Either you follow them more closely or let them go.

As a lead is your job to put them on the right track, so you should try to understand their way of thinking and weak points.

If even after trying hard you still can't get them to perform properly, let them go. It might be that they're too "scatterbrained" or that you weren't able to find the right way to approach them, but, either way, if you've been through, it doesn't really matter.

YMMV, but the point is: you're the one supposed to 'bend' more to make the collaboration work. If you can't, you still should try to solve the situation (for the sake of all parties involved, not only yours) by letting them go, having them reassigned, or something else.