WINSCP disconnect though mikrotik

Dear all ,
does any one knows why i will be disconnect after a while when connecting to a machine behind mikrotik by winscp without any firewall enabled on mik and machine?
i should mention that my Band Width is goood from mik to machine and mine to mik ,
i only have problem only by SCP protocol , i have SRC and DEST nat (two way nat ) from machine to mik and wise ,

does any one have this problem ?

Thank you