Trying Bigger, getting to medium size perler project

I tried to do medium sized project with perler beads and it was a success:

So, I am amble to do more than small project.  I can make bigger project and it hold on.  I must miss something for the extra large project like the one with Wonder Woman.  I may be clueless but hopeful.    Failure must have a reason that I don't see right now.  For now, I will do a few more medium size project to built confidence.  After that, I may make another try for Wonder Woman.