It might've taken too long to stretch out, it seems like the string can only shoot out so fast, and Jolyne's hand itself can hold quite a bit of string(can jolyne also control what part of her body unravels to use as string?), so even if she caught up to foo fighters, with a hand that's mostly unraveled she probably would be at quite the disadvantage. Of course this is all coming from my assumptions of the manga so far, so IDK. Working on taking off the cuff does seem like something Jolyne should have at least attempted.