Combinations Generator: won't let me use only 2 out of 4 available attributes

[dressmylegs.ie]

Hi! I have 4 attributes to choose from: for some products I need a combinations of only 2 of them and for some of all 4.

When I started using the combinations generator first, everything was fine: I could select the values from 2 attributes only (size and colour) and it would generate the correct combinations.

Then something happened, and now when I pick values from the same 2 attributes only, it automatically picks ramdom values from the other 2 attributes, thus creating incorrect combinations. I can still do it manually but with the amount of products in stock, it's very labour and time-consuming.

My developer says that it's a bug in Prestashop that if you have a certain number of attributes created you MUST choose a value from each to create a combination. I know this is not true as I spent 1 week initially creating products from 2 attributes only using the generator and all 4 attribites were created with values at that stage.

Please help!

P.S. I need to keep all 4 attributes with values in place as they are used for other products.

Thank you!