Hello Mats!
Since you are using clusters, the issue here is probably cluster element order. When you are using clusters as array elements, and you want to add a new array element, the new cluster's elements must be in the same order as the clusters in the existing array. You can check and change the order of cluster elements by right-clicking on the border of the cluster, and selecting "Reorder Controls in Cluster..."
Please take a look at my attached VI that demonstrates this issue. Also, for more information, check out the LabVIEW help under "clusters" and "modifying cluster order."
Hope this helps!
Liz Fausak
Applications Engineer
National Instruments