ActionScript 3.0 :: Alpha Only Disappear And Reappear Once

Here's a snippet of code I am working on. I have two buttons symbols, instantiated as 'up' and 'down.' There is a movie clip object instantiated as 'circle.' Up and down are supposed to respectively raise and lower circle's alpha gradually using a timer.

Here's my code:
var upTimer: Timer= new Timer(100,10);
var downTimer: Timer= new Timer(100, 10);
up.mouseEnabled= false;
upTimer.addEventListener(TimerEvent.TIMER, appear);
function appear(e:TimerEvent):void{
[Code] .....

The problem is that I can only make alpha disappear and reappear once, and then the the program stops functioning the way I want. I press down and then up, and it works but then after wards the alpha on circle only goes up and down a little bit. Why?