Collins' issues are probably a combination of things.
Collins has never been a monster tackler. His 66 and 67 solo tackle seasons ranked out of the top 30 DBs in both 2005 and 2006. This season, his tackling is down, and probably more due to supporting cast than anything else. In 2005, Collins played alongside SS Mark Roman and behind WLB Robert Thomas. Last season, he played alongside SS Marquand Manuel and behind an injured Nick Barnett. This season, he has a solid DT unit, a healthy Barnett, and rangy pursuit and run stopping talents in AJ Hawk and Bigby.
Packer homers and coaches have fussed over Collins' coverage instincts many times over the past two plus seasons. His tackling has never been Bob Sanders-like, but it's sufficient. Unless you have elite talent, favorable scheme or a poor supporting cast, though, a free safety isn't going to crack the top 20 tackling DBs.