If you are using Huawei or Asus devices, TimeTree may not be able to send you notifications due to restrictions from your device. In addition, widgets might not display right due to the battery-saving settings.
First, check the settings of your device.
The settings depend on the model number of your device, please see this help page:
Follow these steps: "Android phone Settings" -> "Power management" -> "Auto-start manager" -> DOWNLOADED page -> select 'Allow' for TimeTree.