If the hanging is not successful, the retry is immediate. They will get ready the things and within 1/2 hour the guy is again hung.
Imagine this:The fear is so bad yet have to go through it again!
Read this local book:
Hung at Dawn http://www.selectbooks.com.sg/getTitle.cfm?SBNum=37914which throughly explains the death sentence in singapore.
No, the death is NOT immediate, even though the death cert says so.
The person who is hung is left in great pain and dies only after 20min.
Many times, there is blunder during the process, and this is NEVER reported.
If the rope is too short, the guys head can drop off.