Saving te Value of a variable into an XML Variable

I have a Problem. I use a couple of XML process variables for my Workflows. The problem is that if i try to save a value in a xml variable, its empty afterwards. But I dont know why. Any ideas from you guys?
Thanks for help.

Some more Informations, I have the task to Implement 2 old Bonita 5 Workflows in Bonita 7.1.5. with an external database. So when I write the values of the xml files into text files and write the text files into my database, it uses the default values, so I guess that the values I write to the xml variable dont hand over in the write way, so the xml variable will hand over the default value to the text variable.