No Sync on NSUbiquitousKeyValueStore

One percent of my users have the following problem:


On Device A, an object was stored under

NSUbiquitousKeyValueStore *store = [NSUbiquitousKeyValueStore defaultStore];
[store setObject:@"TEST" forKey:@"keyTest"];
[store synchronize];


On device A you can easily read this object.

On device B, this object is not available under this key. The Users say that all necessary settings have been made on both devices.


Where is the mistake?