Screencapture ~/Desktop/screenshot.png not working

Question

I just set-up my MM and I have a applescript that always runs fine on all my machines.
set dFolder to "~/Desktop/screencapture-30/"
do shell script ("mkdir -p " & dFolder)
repeat 30 times -- Repeat XX times.
          set tTime to do shell script "date +%H-%M-%S"
          do shell script ("screencapture " & dFolder & tTime & ".png") -- Capture screen.
  delay 1.08 -- Wait for xx seconds.
end repeat
However it runs not on the MM and I get a error
after I did a restart I do not get the box, the folder gets created, but no screenshot is made.
If I use this (screencapture ~/Desktop/screenshot.png) command on my MBP (i5) computer in terminal I get a screenshot.  on my MM (Core Duo) nothing happens.  Both run 10.6.8.  Anybody know as to why?    How to fix? 

Answer 1

Sorry here I am again hope you still reading this post RW
I have
set dFolder to "~/Desktop/screencapture-30/" --sets folder
do shell script ("mkdir -p " & dFolder)
repeat 1 times -- Repeat XX times.
          set tTime to do shell script "date +%T"
          set dDate to do shell script "date +%d"
          do shell script ("/usr/sbin/screencapture " & dFolder & dDate & "-time-" & tTime & ".png") -- Capture screen.
  delay 0.1 -- Wait for XX seconds.
end repeat
it works fine on one machine and not on the MM, is there something I am missing again?  I tried both, the path to the desktop and ~/Desktop.  It works fine in both variations on one machine and not on the MM.