(A) To check on the work that someone has done (separable)
1. I have to FOLLOW Sally UP and make sure she did everything right.
2. Please FOLLOW this UP. I want it done right.
3. He FOLLOWED UP his findings by reading books to see if he is correct.
(B) To take appropriate action about something
1. Police FOLLOWED the complaints UP with several arrests.
2. He complained several times, but the police never FOLLOWED UP.
3. The police are now FOLLOWING some new leads UP.