I had the same problem and I have a hint to solve it.
My company have 2 licenses, first we bought the "Standard Server License" to generate documents containing only text (type one). A couple weeks ago we bought the "Professional Server License" cause we needed to generate documents containing JPEG images (type two).
When we generate a "type two" document it shows the watermark, we have the Professional License in the right place. But, if after that, we generate a "type one" document, that doesn't show the watermark, and we generate, again, the "type two" document it works perfect.
The code generating the "type one" document have the Standard License, and the "type two" have the Professional. If I change that, and work in both with the Professional License, the watermark is shown in the "type two" documents ALWAYS.
The only way to generate the documents without the watermark is having the two licenses and following the procedure described before.
I hope this help u to find a solution to the problem. If u have any comment or question please contact me at ebautista@storecheck.com I'm a native spanish spoken so I know my english is not to good, but I'll do my best to help