Linux – df displays 49% free space, but if you try to touch a file, the error message "There is no space left on the device"

a df command shows that 49% is free. However, I do not think that's true. I can not touch files or other simple operations without the error "No available space on the device".
Is there a way to synchronize df without restarting the box? I suppose there is not enough space for df to be synchronized and that is why it generates bad reports.