You may experience one or more of following problems while synchronizing files:

• All files are transferred every time, although only some files were changed.
• Changed files are not transferred at all.
• Changed files are transferred only sometimes.

In all above cases the problems is usually caused by WinSCP not correctly converting timestamps from local conventions to remote server conventions and vice versa.

Also with FTP protocol, the server may not support preserving timestamps of uploaded files. Usage of synchronization in local to remote direction is limited then.

See documentation to learn about possible problems with timestamps.