Blank Thumbnail boxes

For no apparent reason, there are spaces that are blank in any given thumbnail row. Some rows are full with all 4 images, where some only have 1 image on the row. It looks very odd and I can’t figure out why this is happening. If you can help asap, that would be wonderful, as we were supposed to launch the website today.

http://www.leppdesign.com/portfolio/

http://wordpress.org/extend/plugins/nimble-portfolio/