Nimbostratus
SIRT
There are some known issues surrounding XML validation, so it might be helpful if you could let us know which version of ASM you are running, and the definition of the specific parameter (from the WSDL & XSD) that you are trying to apply security to - if you can't publish that here for security reasons then I'd suggest opening a case where you can supply that information in confidence.
Off the top of my head, however, there is this known issue which might apply:
https://support.f5.com/kb/en-us/solutions/internal/9000/100/sol9120.html
We've also seen an issue recently with the way certain regular expressions are applied when they are defined in an XSD - at the moment that is still an issue being looked into by the Engineering Services and Product Development departments so I can't give you a resolution if that is the case here, yet.
As I say, a little more information about your environment and configuration will help us in determining the root of the problem a great deal.
Thanks,
Aaron