that happens from time to time and is not any particular fault of an app, phone, etc.
you are syncing wirelessly and you are bound by those laws, in that being wireless is the weakest link in the chain.
could be environment factors, could be the server for that app is experiencing an issue--could be any number of things.
in short, what you are seeing is normal. what is NOT normal is when it constantly refuses to sync after hours have passed.
ive seen the sync issue on my exchange server. its short term and corrects itself. So do not worry.