I can't see immediately what's wrong here. It would be useful to see what's in the relevant class (e.g. how the function name and arity are defined).

Using the -TJ option on the command line, or the equivalent Configuration option in the API (FeatureKeys.TRACE_EXTERNAL_FUNCTIONS), can help to reveal why functions were not found,

One possibility is that the system has got confused by your choice of namespace URI, which follows the conventions for "reflexive extension functions". It should be possible to use any URI, but there's always the possibility that the code has gone down the wrong path.