Please rebuild indexes and update statistics on the database and the SQL Data Sync may be able to sync successfully again. Try to do these maintenance tasks daily on the hub and member database.
You can also examine long running queries on the "Query Performance Insight" section of the Azure SQL Database (Azure Portal) and identify long running queries that may be related with the tables SQL Data Sync is replicating.
Verify also computing resources consumption is not getting 100% while Data Sync operates. If that is the case, you may need to scale up the service tier of the database.
Take in consideration SQL Server Express is able to use only 1.4 GB of RAM, try using Developer Edition.