script doesnt update when gather fields change

i have a field that gathers info from a couple of other fields and it doesnt seem to updat until i change one of the other fields twice . my filed calculation is the field that seem to have a problem is the field "tax to add" field i have to change it multiple times to get this calculation to work.

var total = Number(this.getField("TOTALPRICETODDLER").value)+Number(this.getField("TOTALPRICEYOUTH").value)+Number(this.getField("TOTALPRICESXL").value)+Number(this.getField("TOTALPRICE2XL").value)+Number(this.getField("TOTALPRICE3XL").value)+Number(this.getField("TOTALPRICE4XL").value)+Number(this.getField("TOTALPRICE5XL").value)+Number(this.getField("tax to add").value);;
if(total == 0)
event.value = "";
else event.value = total;

thanks