Android phone manufacturers listed below kill your apps even when Google recommends that they should not do that.
They make changes to the core Android OS with the expectation that it will improve battery life, but not only are those changes not even very effective, but they also break well-written battery-efficient apps.
— from DontKillMyApp.com.
If you have a phone from one of these manufacturers, please visit the relevant links at dontkillmyapp.com (a site developed and maintained by a group of other app developers) to understand the issue and fix it on your phone.