This simple script only plots SMA, but don't enter although there is a clear signal on the chart.

Anything wrong?

Code:
int period=55;


function run(){

set(PLOTPRICE+PLOTNOW);
BarPeriod = 15;
LookBack = 500;
StartDate = 20121024;
NumDays = 55;

  set(TICKS);

TimeFrame = 1;
vars Close15 = series(priceClose());
vars tmp115 = series(SMA(Close15,period));


if(crossUnder(priceClose, tmp115))
enterShort();

plot("sma", tmp115[0], 0, RED);

}