OK, it seems like your phase is not set correctly.
when your incident wave is perpendicular to the boundary, you can assume that your phase is the same on the boundary and fix it to 0.
But in your case, your incident wave is not perpendicular to the boundary, so you have to define a phase shift in your fortran file. Basically, the phase will be <k,x>, where x is the coordinates of the node and k the wave number.
you can use the example files, as btwi (btwi-par.f file) for instance. The phase shift isgiven in ALFAP variable.
If you still have a problem, don't hesitate to send your case (parameter file, fortran file, mesh) for me to have a look.
Christophe
(sorry : I call .par or .cas the parameter file ^^)