I've seen this get pinged a lot, but you have to remember that it's a fine, fine line between being on and offside. You may have gone for it just as he touches it, but then you might have been offside for the past three seconds. Refs won't warn you about that all the time, because you should know, but also because it might not make a difference. If you do get the tackle it might not be that you went before the scrum was over, but because you were in an offside position when the scrum was over.