I have the same issue since a long time. It very very rarely syncs. I am 90% sure I have the correct settings because it did occasionally do it. Last time now was probably more than 6 months ago.
It quite quickly synced last time i got a new mobile.
It is my only serious concern about this app. I use my phone as the base and regularly make a backup. I never make any changes on my iPad which is due to this issue rarely up to date.
Has anyone else had this issue and found a solution ??