I agree w/ Rabbit and CCK.
I would start going out with her and walking her on leash to "the spot", give the command, wait her out, walk around the spot until she eliminates then praise like crazy and add a little treat just for good measure.
Do this until you get a solid behavior history, then you can try letting her out on her own again.
Sometimes dogs only learn things partially and if structure is taken away too soon the behavior falls off.
(Sometimes dogs are just weird. We have an 8X8 fenced area strictly for pottying and we stand in the doorway and watch the dog potty the entire time. Fairly often (often enough for us to joke about it) my dog will tinkle, turn around and ask to come back in. We either let her back in and send her right back out or we refuse to let her in and we tell her "POOPY POTTY". So she stands there for a few seconds like "seriously? You're not letting me in"???, then she starts sniffing around, then she takes a HUGE poop...and we're like "How the hell did you not know you had to do that"??? LOL)