this is kind of a loaded question, because there could be a number of things going on.
some things to keep in mind:
1. the basic concept of opening the closed guard is to lengthen his legs as much as possible. if somebody's legs are as straight and long as possible, then they can't cross their feet without shortening the length.
2. make sure to get his legs mainly around your pelvis or your ribs. you want him on bones, not gooey stuff. thus slide up or down to get his legs mainly on one of these areas. if his legs are more around your hips, then do the open that you're already trying to do.
3. you need to mix in other guard opens. if you only do one, then he can keep blocking it. it's kinda like how if you want to armbar somebody, you need to threaten to choke him first.
these are the most general pieces of advice i can give. how is your technique on the guard open that you're trying? do you need advice on how to perform that particular one? is the guy really flexible? is he breaking you down everytime you try to apply pressure? are you applying tension to his knees, ankles, and hips simultaneously as you try to open him up?
need some more info