Which operating system and where is the file being ran from?
My line of thought is if you are running on widows vista or 7, windows may be blocking it because of the location of the files.
I don't know how windows vista/7 would, react to Ant in, say, c:\javaProjects\ unless you set the required permissions on the javaProjects folder.
Are you executing the build script from inside Eclipse, or independently from the command shell?
Have you tried executing ant with the '-v' option given (for diagnostic output), and added an
<echo message="Build is '${build}'" />
statement to trace the execution itself?
Edited: Thank to Peter's Loron question I checked workspace log file MyWorkspace/.metadata/.log There I saw error message I googled it and found that this happening because location of my Eclipse changed, but path to ANT jars remains old.
TO Fix that you have to go Preferences->Ant->Runtime and click Restore Default it will pickup new jar location and warn you about absence of tools.jar, copy it from somewhere and drop it in same folder where other ant jars.
I'm using IBM RAD7 (which is based on Eclipse) and I was having exactly the same problem. After much fiddling I went to "External Tools" | JRE Tab | select "Run in same JRE", mine was set yo use the Webpshere JRE. Took a day to fix it! Nigel