Any Idea What causes Image Pack Numbers to Fluctuate So Much

Apologies if this is question is not in the right place...But I have a site that is moving between 12 and 1 image packs on a regular basis. The odd thing is the ranks aren't moving for the keywords and the site ranks in the number 1 spot in google for over 40 keywords. So is it that image packs don't always show up for the keyword tracked? Or is it something else? The image stuff has a lot less info in regards to answers to these types of questions so any help is greatly appreciated