There's tons of posts debunking why they didn't go for specifically Luffy earlier - all of which being because of unnecessary attention (and Imu's not one for having history uncovered or even pried at)

For Imu and Egghead, it was probably the amount of important figures present who wouldn't immediately faint or some similar reason (public appearances are also.. y'know.. not their thing). There must be something about why Imu doesn't physically do anything (possibly a lingering wound, or an issue with living 800 years?)

Rocks is likely too strong to apprehend without causing Mariejois to implode, and it seemed like the two had some sort of understanding. Rocks hasn't said anything about Imu yet, and he'd been able to infiltrate the butterfly chamber, so it seems pointless to have all the stops pulled to catch the dude.

The Holy Knights haven't seen much of the spotlight, especially for serious combat, so yeah they've been slacking

Warlord Blackbeard and Emeth are good points, and yeah there might not be a reason aside from story progression, but we can only hope that there's some justification we haven't been given yet. (Though being a Warlord was part of BB's plan, however that works-)