You're gonna need to keep at it...I've read that the seeds from the stilt grass
is viable for 7 years...I'm facing the same situation and plan to mount a full
attack with my sprayer tomorrow morning when it is supposed to be cool
(relatively speaking!)

Ornamec is wonderful but be sure to fully read the directions before
using...also, to my nose, the smell is awful. But, the stuff works and I really
haven't seen that it harms my other ornamentals. Of course, I've not used it
near ornamental grasses.

Another word of warning, snakes love to frequent areas infested wth stilt grass
so keep your eyes open.

Marcy