why symbol s animation changes doesnt show in main timeline?

Are you using Movie Clip or Graphic symbols? 
Movie Clips will NOT show their nested animations unless you double-click to go inside of it and play the nested timeline. MCs WILL play when the document is published however.

Graphic symbols WILL play their nested timeline(s) inside the Animate authoring environment because they are frame-based and in syc with all other timelines including their parent timeline.

Now, with Graphic symbols, you have the option to control how it plays by selecting its instance and in Properties, select Play Once, Single Frame, Loop, etc. There's also a Frame Picker panel dedicated to even more granular control of Graphic symbols.

So without seeing a screenshot of your FLA, it's hard to pinpoint exactly why you are hving this issue, but I bet its simply how confusing the differencve between MCs and Graphic symbols can be if not familiar.