Android System Power Management
Starting with Android 5.0, many devices will act aggressively to try and save system battery power. Unless certain steps are taken, Tasker may not function as expected, either because Android is not sending it the needed signals in order to save power or because Android is even killing Tasker.
A commonly noticed symptom is that profiles with Time contexts don't activate or deactivate at the expected time.
Make Sure Tasker Is Not Killed By The OS
Making sure that Tasker is not killed by the system can be tricky because of all the restrictions Android applies to background apps, but in the majority of cases it's possible to do.
Please go through the list here for Tasker specific settings.
Also make sure to check out this page for vendor specific procedures: dontkillmyapp.com