Every theme is a little different. I’ve found this is a fairly common problem. First try making a gallery with no titles. Do the thumbnails line up now? If so try and limit the length of the title or make each title roughly the same length (see example) and the problem might go away.
https://www.be-awesome.org/exercises/back/
If not, try changing the size of the thumbnails which if you don’t change anything else will change the spacing between the thumbnails. Or try reducing the text size. Also try reducing the number of columns from 3 to 2.
If all else fails you could change the width of the page. While most themes (for crazy reason that make no sense are fixed width) they don’t need to be or be the size they typically are usually just around 960 px wide.
How to do that depends on the name of the elements in your theme.
For example in the theme I’m using:
/* control width of page as a percentage of whole */
.pageWrapper, .top_menu_bar { width: 64% !important; }
The above little snipple of CSS sets the width to a value which can be all the way up to 100% which gives everything more room to fit.
You’d need to ask your theme author which elements are involved, I doubt their the same as above.
