Sometimes the Wallet-APP in Windows, Mac or Linux need a long time to sync the whole blockchain.
In this case you can manually add some Fullnodes with a high and stable Internet Connection to your Peerlist to give the sync a "boost".
To do this, open the CLI in the APP and type the following Command:
addnode "ipaddress:port" add
Click Enter and and when the output is 'null', the command was successfull.
A list of Fullnodes can you find here.