There are several reasons why this issue is occurring:
1. lack of memory space to save the progress (RAM)
2. playing the game offline
3. device is no longer compatible with the app and does not meet the minimum system requirements.

❗Minimum System Requirements are:

🎯Android:
- more than 4GB of RAM
- free space of 200 megabytes on RAM

🎯Apple:
- more than 4GB of RAM
- free space of 200 megabytes on RAM
- iPhone 8 or later, iPad 5th Generation or later.

❗RAM is different from internal memory (storage) and usually varies between 2GB ~ 18 GB. It stores the "frequently" used items so that the processor does not have to go all the way to the internal memory and then find the item the user has requested for. 

Check the instructions below on how to clear the RAM: