Model stifness matrix singular

Hi there,

I'm trying to model a simple beam model structure. My first test without any defined joints solved. However when I add a few joints in model I get a sigular matrix. 

I want elements 3, 5, 7, 9, 12 and 13  to be able to rotate freely around the y axis at nodes 2, 3, 6, 7, 10 and 11 respectively. The last 2 conditions, for elements 12 and 13, doesn't give any problems, but the first 4 does.

Any help?