My experience is not to attach the top container while the bottom container is on a flat surface. I would suggest that you hold a container in each hand and seat them together that way.
If not careful, the pins that join the containers together can be pushed back into the container. The pins resemble small "nails" driven into the bottom of each container from the inside so that the head of the nail is inside the container. On the new ones I just received I found that it was hard to get some of the containers to fit together. I made the mistake of placing the bottom container on a flat surface and applying pressure to the top container to get it to seat. This resulted in some of the nails on the bottom container being pushed back into the inside of the container so they no longer protruded.
The nails have a very snug fit which really secures the top container. Just makes it a little difficult to get them apart when you want to separate them. I pulled the bottom off on one container while separating it from the container it was attached to. The bottoms of the containers are not really "glued", but appear to rely on a little bit of paint (maybe it's dabs of glue) to secure them. Once I removed the bottom I was able to push the pins back in. However, once the bottom has been removed there is no paint (or glue) to hold it securely so the bottom is just held by tension with the side of the container.
