Problems with variations

I use this plugin on my site (https://www.slingersenzo.nl) where customers can design their own t-shirts. I use 2 variations: Size and Color. I have 5 sizes and 6 colors for men shirts.
In the plugin I have 3 parts: Front back and canvas (I use this for an other product). When I go to “product parts” on my men shirts, I only applied a background image (included) for the Front part and a background image (included) for the back part. I do this for every product variation, 60 images in total. I checked the checkoxes for all the parts I use.
When I save, at the last 3 product variations, the background images are gone and the checkboxes are unchecked for both the front and back product part. No matter what the order is of my variations, allways the last 3 product variations are handled this way.
The same thing goes for my womens shirts.
Is this a restriction in the free version or is there a setting I can change so I can use all of the product variations?

I use version 2.0 of this plugin.

https://www.ads-software.com/plugins/woocommerce-products-designer/