Google Analytics shows avg. session, sometimes 00:00:00 sometimes not 0

[Forgivetr]

I integrated google analytics into my android game. Until today, there was a no problem. I can see average sessions normally until today. But for today it shows 00:00 for 7 sessions also I noticed that sometimes it was 00:00 before today for some sessions. Why does this problem occur? What is the main reason? How to solve?