I think I have a bit of a handle as to what's going on here (note, no inside info, just speculation).
1. Incoming OPP chief gets handed the file on Day 1 in office. Trust me, this guy wants Ford's head, but also doesn't want investigation to collapse. ALSO wants to show that he's a new broom, independent and so on. A classic recipe for throwing the file back at TPS and asking for further and better particulars.
2. In particular, if the argument is that Rob and Lisi thought they were being blackmailed, why aren't you charging the blackmailers? "Oh, we don't have evidence for that". Well, there goes your motive. "But all we need to prove is they THOUGHT they were being blackmailed." Nuh, uh, if you don't go after the blackmailers too it looks political.
So TPS are frustrated because they feel they have RoFo bang to rights, which they do, but New OPP Guy is feeling his power and also determined not to be made to look like an ass. Wants the to go and do more homework. After a long investigation like this which they thought was over bar the shouting, very frustrating for them. But the kind of thing that happens all the time in other arenas.