Google’s John Mueller said in a Google Webmaster Hangout video that there will probably not be another Toolbar PageRank update ever. His exact word were -
We will probably not going to be updating it [PageRank] going forward,
at least in the Toolbar PageRank.
So in short if your site is receiving a lot of traffic i.e search traffic but rank is still showing zero then most probably internal rank is high but tool bar is showing zero because its not updated -
Check the complete video,