If you have this problem, you might check the following steps:
1- Have you enabled the Background Refresh for the app.
2- The phone isn't in Low Power Mode.
1- To check if the background refresh is enabled, open Athan Pro app --> open the Menu then click on Settings. Scroll down to the bottom and click on "Check Location & Notifications". You will see if the Background Refresh switch is ON
2- To check if your phone in Low Battery mode, simply look at the color of the battery gauge if its yellow or green. Yellow means that your phone is in Low Battery Mode, green means normal mode.