After a big heist, a problematic cop who spent years undercover in a gang, must come back to active when he finds out that the criminal that he’s always pursued is still loose.

Agreed with Richiev and DPG.

Also, why now? What is the inciting incident that motivates him to catch the criminal? Did the criminal do anything SPECIFIC to necessitate the cop to take action?

As Richiev said.

And as cops are always pursuing criminals on the lam, what makes this one stand out from all the other criminals in his case load? ?Why must he focus all his time and energy on this bad guy when there are so many others he could be pursuing?

You buried the lead.

"When he discovers the criminal he's always pursued is still on the loose, a former undercover cop..."