Issues with set 10234 book 3 pg 43
Sometimes, the piece connecting your two white plates in Step 2 might be bent or not completely flat. An rounded LEGO piece can have other pieces that it connects falling off because it can't hold on to the studs as usual. This could happen to the white wing piece itself.

Two, they can be a hassle. Normal pieces can sometimes just be a pain in the back. I got a set with pieces in perfect condition and they still come off. Make sure you squeeze those pieces really tight to each other to prevent and pieces from falling off.