Icloud sync not working for realmac software app: clear to do list

hi i have two phones logged into the same apple ID, i use interchangeably and want to be able to access the same clear app lists and for any changes to reflect on both devices regardless of which device it was made on. for some reason although icloud works perfectly on one device, the second one refuses to sync. i’ve deleted and redownloaded the app on this second phone, cleared out of any lists before selecting icloud and it still hasn’t filled up with my lists from the other device. is there anything i can do?