disappearing airtime

Greetings I would like to say I have a Samsung galaxy note 3 handset and I normally by data using the ussd *129* and then I get the data double I have always a positive airtime balance but lately I found that having data left over a 100 mb then the sms would come and say you have 15mb data left and recharge but when I see again the sms reads that my data has depleted when I still have data left I don't know why my airtime are being deducted.