Walls of Cuisenaire rods

We've been using Cuisenaire rods to build "walls". The rules are, you can only use one colour in each layer of the wall, and it has to fit exactly. First of all we tried it on the white board, using the nrich Cuisenaire environment:

Every number that fits in the wall properly is called a "factor" of the big number.

Then we tried it with the rods themselves, choosing our own numbers. Here we are recording what we did (sometimes we didn't even need the rods).

All the different numbers had different walls. And we noticed that some numbers only have themselves and one in their wall. One person knew what these numbers were called:

Here's some of our recording of what we did: