Been there.... done that.
When you operate the key that locks the seat, a tab moves in and out to lock the back of the seat down. With the seat off, operate the key a few times - you will see it. Notice that the latch is a flat metal tab.
Now look at the slot in the original Honda seat that goes into the latch. It has a matching rectangular hole.
Now look at the slot on the Travelcade seat. The bottom of the hole is not square, its rounded. Those rounded corners don't permit the latch to slide into the hole unless the seat is pushed down way too hard. If you take a file or dremel type grinder, and open up the rounded corners in the slot, to make them square, the latch will then work just fine.
I really don't know why Travelcade continues to do this. Pretty silly.