nsExec::Exec problem & ExecWait

hi,
could someone give me an explanation why some commands don't work if nsExec::Exec is used, but works fine with ExecWait?
i met this problem with xcopy:

variables:
$DIR=c:\source
$destdir=c:\destination
$Failext=*.dbf

nsExec::Exec '$SYSDIR\xcopy.exe "$DIR\$Failext" "$destdir\" /S /I /G /H /R'
ExecWait '$SYSDIR\xcopy.exe "$DIR\$Failext" "$destdir\" /S /I /G /H /R' 

any idea?

tnx