IMPORTANT TIP! Is it actually clogged?

Here is an interesting scientific document on the issue:

http://www.google.com/url?sa=t&rct=j&q=de%20jong%20et%20al%20%20-%202006%20-%20air%20entrapment%20in%20piezo-driven%20inkjet%20printheads.pdf&source=web&cd=1&ved=0CE0QFjAA&url=http%3A%2F%2Fwww.lorentzcenter.nl%2Flc%2Fweb%2F2011%2F466%2Fproblems%2F4%2Fde%2520Jong%2520et%2520al%2520%2520-%25202006%2520-%2520Air%2520entrapment%2520in%2520piezo-driven%2520inkjet%2520printheads.pdf&ei=PWr4T9bXFsakrQG_6M2LCQ&usg=AFQjCNF3cbyOMT5oBfUnQDpTUy4aqEOOnQ&cad=rja

or just google:

"de Jong et al - 2006 - Air entrapment in piezo-driven inkjet printheads.pdf"

This document isn't too bad, but there are some others that are truly mind numbing...
If you google to above, you'll get links to those also.