SAC: unable to sample section of surface

[millsy.g]

L1&P2&P3: Unable to sample section of surface.  

I am getting this error at -some- of the stations along my corridor when using my subassembly. 

The surface it is targetting is fine when being used by any other corridor or assembly.

Very simply, this is a topsoil stripping assembly that we are trying to use such that it determines if the grade is existing, if yes, it starts at 0 depth, then gradually goes to the maximum stripping depth at a width inputted when the assembly is used. Then carries on to either the maximum width, or whatever target is there.

There is also a "start" target which I'm using feature lines for. 

See the attached assembly file.  (Change the file extention from .pat to .pkt, it wouldn't let me upload it)