Why some instance on stage are "null" while others are not

Hi guys,

I was just doing a menu system for a game project, as3 with Flash CS5

The navigation is build on the frame on the main timeline.

When I was creating stuffs on the 3rd frame, the btns on the frame can not be called, returning "null", this is weird.

With further study, I found that, if I delete a instance on frame 2, the problem on frame 3 will be solved. Still weird, huh?

Then I found that giving the instance on frame 2 a name could also make things normal. What is happening?

Why having a unnamed instanced on the previous frame could make the instances on the next frame "null"?

I dont know how to upload fla file here, if you know please tell me, I will upload it and you can play with it.

If you are from official, I need an explanation why I have to find out this bug with 4 hours wasted.

If you are an user, share your opinon.

Anyway, thanks in advance.