Many times I have played this game already and every time I have noticed that whenever I try a heavy attack on the body, nothing changes and the guy never loses steam.

Doesn't matter what his Body Rating is, sometimes it's low, other times it's high, I get the same result. My boxer is a very hard hitter.

So, what am I doing wrong? Has anyone else experienced this?

I love body combo's. They work really well. He might be blocking them. Punch upstairs first , then to the body. Also try different angle and punches.