For those of you who have assembled a Kettler trike - can you please help??

We just tried to assemble DD's new Kettler Alana Trike this afternoon, and it seems that all of the connections that have closed nuts (including the 2 to attach the seat to the base) are really loose and can't be tightened further... any thoughts about what we might be doing wrong?? We put a washer on each as directed, but it's still loose... did this happen to you? Or is there something we could do differently?

Usually we're pretty good at putting things together, so I'm not sure what's wrong (or if we have defective parts??). Of course Kettler Parts is closed today for Good Friday and doesn't open again until Monday.

TIA for any help!