backup via cron partially failing

[Roger Searle]

Hi, I have a couple of mysteries for which all I have are my 
observations and no clues on how to google.  First thing is my 3 backup 
scripts which are simple shell scripts to create tar.gz files of certain 
folders, run by cron.  One each for home, data, and email. 

When run from the command line, they execute fine and I get the expected 
file sizes of several hundred MB or a GB.  When run from cron, they 
consistently create tar files of around 5 or 10 kb.  I can open the 
tar.gz files and see files within, but only a small number of what I 
would expect.  It is as if the script executes then after a few seconds 
it quits.

Can anyone suggest what might be causing this or where I might 
investigate? 

Cheers,
Roger