It might simply be that she is not in the GRO BMD index.
My brother-in-laws Grandmother born in 1916 is not in the BMD Indexes, the entry that is in there is wrong and their is no corresponding certificate.
Therefore I ended up contacting the local Registry Office (Warrington in this case) and given her name and approx birth year and father's name (we had her marriage certificate) they were able to provide a copy of the relevant Birth Certificate where the GRO could not. The GRO's reply when asked why she was not in their index was it was probably a mistake at the time (also seemed to indicate this was not a uncommon problem) and they do not carry out any corrections to the index.