Weird posts positioning

Hello,

I am helping a friend finish a website and there seem to be a problem on a page showing posts. If the number of articles displayed is dividable by 3 (3,6,9) the design is doing fine but if there is another number, ei 10 articles, the design acts weird and there are blank spots where there shouldn’t be. I’m not sure if I’m explaining it correctly so perhaps it is easiest to see the link https://psi-svet.si/psivet/category/vzgoja-solanje/

I tried finding the solution on my own. The only one I found was to set the max. number of articles to 6, but with that code the loop of category’s display got broken.

Could anyone help me out?

Thank you!