Airtel — airtel data pack expiring in less than the promised period

Hello i am a customer bearing cell no-[protected]. Every time i go for a special recharge of rs.169/ for 28gb data as well as free std and local calls valid for 28 days, i find it gets expired in only 10-12days, it is really disgusting to see how the pack whose validity is for 28days expires in just 10/12 days. Kindly explain me the exact reason for the above matter.