Hi there, Yes, when Google checked that your site is not getting traffic and you are not following any rules of Search Engine then the PR of your site can decrease.
If PR is decreasing means, you might have done something wrong in your site., # duplicate content # spam links # black-hat techniques., etc.. Follow white hat techniques to escape from Google's eye..
if your site is search engine friendly, your site importance increase in search engine view and it increase your PR. but you not follow search engine rule your pr goes down.
PR increase or decrease depend upon your site.its search engine friendly or not. If you want your pr increase be sure your site is se friendly and creat quality links.
Basically PR is a measure of your labor. If you update consistently, and keep on getting new links to your blog, then the PR should not decrease. Even if it does not increase, it should remain the same. If you double your effort in updating and triple your effort in getting backlinks, PR should increase.
Because your URL's inbound links come and go... because pages that link to your URL have inbound links themselves that come and go... because the number of outbound links on pages that link to your URL increase and decrease over time... because the number of outbound links on pages that link to pages that link to your URL increase and decrease over time... because pages that link to your URL get indexed and sometimes deindexed... because pages that link to your URL have inbound links themselves that get indexed and deindexed... because Google's algorithm for calculating PR changes over time as they tweak quality modifiers... There are LOTS of reasons your page rank will fluctuate.
PR increase or decrease depending on the value that Google given to each links you have and other criteria to compute for your site PR.But I think link has direct effect to your PR.