Please Help!

I have a main script (ksh) where another script is called (convert_picture). Normally this works ok, but since some changes has been made on the unix-server (I dont know what ) suddenly it doesnt work anymore: i get an error message: ksh: convert_picture not found. I am logged in as the owner of all the scripts (and tools) so the rights are ok.

The line in the main-script looks like this (the script is been called wihtout sh or ./ and this usually worked ok):
convert_picture original_picture.jpg thumbnail.jpg

and the content of convert_picture (here a tool called Alchemy is called):

nice alchemy -j --y -Xb124 -Yb88 -+ -Q -o "$1" "$2"


What I don't understand is: I can see the file with ls of the vi editor, but when I try to execute it, it says 'not found'

Thanx in advance