Why Samaritan is not able to find the connection between Detective Fusco and John & Harold.
Fusco is on the radar but there's a difference between being aware of someone and assigning them as a threat. For the most part, Fusco is just "interacting" with the Machine Gang by encrypted phones etc. and going about his normal job as a NYPD detective...or at least so it appears.
More Importantly - Fusco doesn't know about the existence of the Machine or Samaritan until Season 5.
It's only when he begins an investigation into the suspicious rise in missing persons that, unknown to Fusco, Samaritan has arranged, his activities are viewed in a new light. This leads to Samaritan labeling him as a "possible obstructionist". (S5 Episode - Shotseeker)