Packrat strange behaviour

Hello all, and thank you Yannis for this great plugin !

I am making a definition to pack a number of objects (slices of topography for a physical model) onto boards. 

I am running into some trouble:

First, any idea why one of my pieces comes out as an "element that did not fit", considering I have plenty enough containers? In my definition it is part number 30 of 42. So neither the first nor the last. This is very problematic here as some pieces are disappearing from the result.

Second, it is a bit strange that Packrat starts packing elements well, one container after another, but suddenly skips containers. The only way to have it right is to have the exact number of containers for the elements (in my case 6 containers).

This last part leads me to wonder if there is any way to decide on the number of containers depending on how many Packrat would need ? i.e give a base container and let Packrat multiply it if need be in order to fit all the input elements.

Any help would be very greatly appreciated ! Thanks!

By the way, I am using the Educational version of the plugin.