My pending order is tracking related to present price


Hello everybody, I have a problem with my Expert advisor 

For example : If I want to put Buy-Limit in step 5 Pips of the present price

the problem is when the price changes ,the position of Buy-Limit will change to make the same step (5 Pips)

my Code is 

trade.SellLimit( LotF       ,     Bid + .0005,         _Symbol    ,    Bid + .0065, Ask        ,NULL       ,0     );   

while :

double Ask = NormalizeDouble(SymbolInfoDouble(_Symbol,SYMBOL_ASK),_Digits);
double Bid = NormalizeDouble(SymbolInfoDouble(_Symbol,SYMBOL_BID),_Digits);

I hope I find the mistakes in my codes

Best regards