You would possibly need to post your events so we can see what is happening
Other ideas to investigate could be :-
-are the objects spawning ...or are they existing objects?
-are you using layer names or just default numbers...?
If you run out of options you could place a loop that says....every tick -->place objects on layer
but this would be by no means anywhere remotely near a perfect solution...its always far better to find the cause than treat a symptom
also what z order logic are you using?..your custom logic or someone else's?