file transfering problem

Hello,
I am using filezilla about 2 years, but in last 3 months I have a big problem. I don't know what can be wrong but almost 10th file which I set to transfer always transfer 0b(empty file). It's really annoying if I have to transfer hundreds of files and then I am finding 10% of files empty..(and it's not showing any errors, all transfers are always "successful")

Thanks for help.