Grid is ordering vertically rather than horizontally?

Hi,

I have installed your plugin for a client, it seems to do what we want which is great, but it seems that it is arranging the grid items vertically rather than horizontally - so at this moment in time when I only have 2 grid items, one is sitting above the other, rather than them being beside each other. If I add a third item then that sits at the side. Can you let me know if I can change this, or indeed if I have done something wrong? Many thanks, Nick