MTN data and connection

Hi, I need help, I have problem with MTN SIM card, three days back I purchased 10 GB anytime data and 20 GB night data. I have used it maybe for two days. I have been struggling with internet connection ever since from yesterday, even if signal is strong it won't connect but sometimes it just connect few seconds. Moreover, when I check my balance it only displays anytime there's no night data available on my balance. I'm a student,my SIM card was changed from prepaid to postpaid but it was working fine, it justed yesterday. At first I thought the problem is with my phone and I inserted another MTN SIM card then it worked fine. I just don't know what went wrong. Thanks in advance.