One thing I noticed about the Staggs compilation, is that almost all of his impact came when he squared up the defender, and instead of waiting for the defender to come to him or trying to evade them, he actually attached the defender himself, big fends and pushing them off balance so they couldn't make an effective tackle. He made Origin so I guess if teams did their homework, they would watch footage of the high profile players first and make plans. I haven't tortured myself by watching any replays, but it's possible teams noticed this, and those defending him were instructed to wait for him to come to them, and instead absorb the impact, and wrap him up, effectively letting Staggs initiate the tackle himself?
Like any player, after a couple of years, you need to keep adapting and changing it up to keep the opposition guessing. Staggs has never been a big metre eater, he's effectively an impact centre, and even at his best it wasn't the quantity but the quality of his plays that counted, it's up to the coaches to work with him on how he can use his strengths in different ways other than fend & push.