Yeah, I have a fix, I will email it to you.  It does that if your GL Account descriptions have lower case text in them.
 
What the program does is when you enter the GL Account, it displays the account description so you can verify that you have selected the correct account.  You can then change the line description if you want.  If you do not change anything, then when you advance to the Amount field, the description from the header of the voucher will pull into the line description.  
 
What was happening is the voucher program converts the GL Account description to upper case so when you got to the Amount field and the program was comparing the description to the GL Account Description, if your account descriptions had lower case text, it wasn't an exact match so it assumed you had edited the line description and didn't pull in the vou cher description.  The latest version I have converts it all to upper case before comparing.