In this instance, if, she was found, where I am thinking that she was found, then it's very possible that they just did not see the evidence of the burial site, and the dog(s) did not hit on the grave site. Search dogs, like humans, are fallible. I've my own thoughts about their use, but will save them, for another, more appropriate, thread.
The property was not searched, the first time, until January 22nd. She disappeared after leaving with the men, on January 4th. That's almost three weeks, and, again, if, the remains were found, where I believe they may have been found, then the location, the weather, and the length of time, may have played a roll in LE not finding her the first time.
I've read where the teams who use cadaver dogs, have walked right past remains, and not found them. Later on, someone else will stumble upon the remains, while out hiking, or farming. Some bodies do not emit the chemicals that search dogs are trained to detect, so that could also have played a role.