When is it considered an issue?

My son started kindergarten in September, and very early on he started to mention the same kid keeps pushing him. I kind of let it go because I figure all these kids are new to going to school and it's a hard adjustment. I told my son to make sure he tells a teacher when it happens and to not push anyone himself. Well he keeps telling me this kid is still pushing him, it seems to happen quite often but everytime I ask him if he told a teacher about it he says he didn't. Yesterday he comes home and he's limping a bit and complained that his knees hurt so I asked what happened, he tells me the same kid pushed him. Now I'm really pissed. I ask if he told a teacher and this time he says he did, I ask what they said and he tells me they said that's not good. At this point I know I need to talk to a teacher about this because it's happening alot and now he's hurt because of it (he's a tough kid, 90% of the time if he falls or whacks himself on something that looks like it would really hurt he gets up fine and laughs). This morning when I dropped him off I brought it up to the teacher and they basically said if it was an issue they would let me know and that yesterday he told them and they talked about it. From the sounds of it they never noticed anything before yesterday yet this has been happening almost every week since school started. They also said they are both territorial over the sandbox and my son tends to be close to others and the kid who pushes needs more space. I can understand if this kid has autism or something were he's more sensitive to things but that doesn't mean my son should be pushed around to the point he's being physically injured and no one ever notices untill he speaks up for himself. The teachers have told me about scraped knees or little things like that but yet they never seem to notice this kid always pushing my son down. How much more does this have to happen before they consider it an issue they need to bring up to me?