Many of your friends on Whatsapp might have disabled Last Seen option, so that you can’t track them online.
So here is simple trick to know if he/she is online or offline.
Watchdog is cool app for android which will spy on your Whatsapp friends. With the help of this app, you can know whether your boyfriend/girlfriend is cheating you or not. Watch dog will keep all records of log out/last seen time even if the contract has disabled it.
Download watchdog for android