Sometimes the files does not be transferred and we don't find a log error

We use WinSCP for SFTP transfer but we have some not consistent problem, some times the files are not transferred to de target.
I have 3 questions.
1) it is possible that the error is the size file?
2) exist a version that we can use as a company?, some version to buy?.
3) if exist a version to buy, could you plese send me the differences between them?.

thanks in advance for your attentions.

What do you want to achieve by buying WinSCP? We do not provide paid support, if that's what you are after.

If you want to seek a help with your actual problem, please read how to ask for support or report bugs efficiently, so others can help you.