Ranking for one keyword dropped

Hi Guys,

Not sure if this is the right place to ask but here goes. My website stubbycoolersonline.com.au has been live for almost 10 years and has been re written 3 times in that period and is currently going thru the fourth. We have several major keywords and phrases that we rank in the top ten for consistently however recently one of our major keywords dropped out of the top 100 after ranking 6 to 12 consistently in the said time frame. The keyword "stubby holders" (this word/phrase is one of our rainmakers as well) has ranked well along with many other keywords for the home page. I have looked at my google analytics and it shows average of 12 right now, with no errors or issues from Google, and occasionally when I do a manual search it shows up on page two but rank tracking software shows out of the top 100 so I am a little confused as to what might be going on.

Can any one shed some light on this as I really don't know what the issue could be and yes I know that the current site is not perfect but it has and is working very well except for the one short tail

Thanks in Advance