I stopped working on X-Cart for several months due to several seemingly "unexplainable" errors and have recently started again on version 3.4.14. I'm still having the same problem with thumbnails displaying in 1 column!
The only things I have done are to change standard details in the admin section (shipping & company info, etc.) and add the "Enable/Disable Profile Modified Email from Admin" mod.
http://forum.x-cart.com/viewtopic.php?t=3410
Under General Settings > Appearance Options, and I have 3 as "Display products list in multiple columns". I have 7 (test) products displaying all in 1 column.
Has anyone else experienced this? Since the suggestions below did not work, what else could it be?