V.W.
Have you ever used a command with her when you walked her? We taught our dog to go on command. It doesn't matter where she is. When we say "hurry up" she knows what it means and is her signal to potty.
Also, you may need to put the leash on your dog and walk her out onto the sand. Have you tried doing that, to see if it helps? When my dog is reluctant to go I put the leash on her and walk her in a circle, giving the command "hurry up". She continues to go around in a circle until she stops to go. Works every time. (walking stimulates the bowels, so that helps)