We had the same problem with our more active fuzzy moving the lower ladder
to suit her.  She would push it to one side until one of the hooks was
stickingout and subsequently the ladder would fall in a few moments.  After
seeing this a few times, my husband took a small soddering (spelling?) iron
to the lowest platform (where the ladder hooked in).  We moved the ladder
in (about 1 inch) towards the other ladder that goes up to the second
platform.  He then used the soddering iron to make two new holes for the
hooks to fit into.  These holes are a little bit smaller than the original
holes, but still allow for removal during cleaning.  But we have not had a
problem since then.  The fuzzies go up and down and all around and the
ladder has not fallen once!  Hope this helps!
 
Marsha, Chris & the 2 sweeties (Poto & Koani)
[Posted in FML issue 2730]