An ex convict must rescue his granddaughter from sex traffickers who might be a part of a prostitution ring.

The structure should represent, the Main Character, an Event and his Action in response to that Event.

This is a standard formula which works in almost every case.

I'd get rid of the "who might be a part of a prostitution ring" part.

INTENTION: rescue his granddaughter.
OBSTACLE: sex traffickers.

It lacks a hook that separates it from "Taken".

How about...

-? An escaped convict must rescue his granddaughter from the same sex traffickers, whom he inadvertently helped to break free. (I'm not sure if the hook/twist is strong enough here.)

-? When his granddaughter is taken by sex traffickers, a convict, a former FBI agent, is given 24 hrs to find the men responsible. (We've got a ticking clock/deadline.)

-? When his granddaughter is taken by sex traffickers, an ex-con returns to his old ways to find the men responsible. (Very tired and cliche.)

- An ex-con must return to prison and kill a criminal mastermind or else sex traffickers will kill his granddaughter.

(This is similar to BRAWL IN CELL BLOCK 99. Which may I add, is a pretty cool film ha.)

Admittedly, none of these have that strong hook - but I'm spit balling.

Good luck!