Improving Masonry layout

Hello,

I’m using Nextgen Gallery with your great add-on, but somehow the Masonry view is kind of weird. Some images are next to each other, others are on another ‘line’. I don’t know what causes this, the theme, WordPress, Nextgen or the filter option, but I would like to see the page as compact as possible, so as little whitespace as possible. Do you have a clue how to do this?
The gallery with filter is live visible on http://farfarm.nl/gallery-nieuw/

Best regards,
Marcel

http://wordpress.org/plugins/nextgen-gallery-powertags/