Your Liquidambar 'Gumball' should have produced lots of healthy leaves by now, and to me the most obvious reason why it didn't is that the tree could be planted too deep.
Normally trees should be planted at the same depth as they were growing at the nursery because they need their uppermost roots to be just under the surface in order to get enough oxygen. Even if it's planted only a few inches too deep the tree will struggle. Please check the depth of the uppermost roots.
Another possibility is that the trunk has been damaged in any way. Could you try and upload photos of the situation? I would also like to know what soil the tree has been planted in.
BTW, I'm also living in the Netherlands so if you're not too far away I could come over and check your tree out if you would like me to.