Shoot, well I’m not entirely sure then. I do know that the Invoke BO method widget is notoriously frustrating to use, but it appears to me that you’re doing everything correctly i.e. filling the dataset with a getbyID method, modifying the appropriate fields (including rowmod as you stated) and then invoking the method.
The only other thing I’ve seen on here is filling the dataset using the “fill table by query” widget as it might be mapping the fields more appropriately.
I tend to do most things with code rather than widgets, so perhaps someone who’s more widget savvy might be able to guide you on this one. Sorry I’m not being more helpful!