Help Google Analytics shows avg. session, sometimes 00:00:00 sometimes true

[Forgivee]

I integrated google anaylitics into my android game 5 days ago and published. Yesterday, I noticed that many sessions'(almost half of all sessions) duration is 00:00:00. Rest of sessions seems normal, no problem. My game has just 1 activity, it uses swipe tabs.
What is the main reason? How to solve?