I'm not sure that you mean by "they cannot delete again".
Though in any case, this does not look like WinSCP issue. Unless, it works differently in other clients.
Consider asking your question on more appropriate site, like Super User.

Dears,
I am using WinSCP library to upload a set of files.
The files are being uploaded properly and users can read them.
But when they try to delete the files (although they have permission to the folder from IIS) when new files are being uploaded, they cannot delete again.

I granted :
Perm.GroupRead = True
Perm.GroupRead = True
Perm.OtherRead = True
Perm.OtherWrite = True
Yet the problem persists.
Any advices?
FYI, for other places which they use Bitvise as FTP server, it works fine

Thank you
-Alireza