Hi Lesley,
I see from several other posts of yours that you have a great deal of information about this family; it is a good idea to indicate this when you post a request, and to provide all relevant information - it would help others to help you.
From the other posts, it seems that George SCOTT m Isabella ARMSTRONG (this is in the IGI), 19 Feb 1869 at Canonbie, Dumfries.
Three of their children are in the IGI (the IGI extractions from the Statutory Register stopped ca 1875):
Helen SCOTT b 10 Dec 1869 Castleton, Roxburgh
Ann SCOTT b 13 Oct 1871 Castleton
Mary Catherine SCOTT b 26 Nov 1873 Castleton
Elsewhere, you indicated that Ann b 1871 was your ancestor, as was her daughter Elizabeth b 1885.
So presumably it is Annie aged 3 in the 1891 census whose birth you can't find? You have not given the full details from the census. Is she listed as SCOTT in the census? Is she listed as a daughter of George and Isabella? And, very importantly, what is her birthplace as given in the 1891 census? Also, what is the birthplace of 7yo Janet as given in the census?
Have you found the family in the 1901 census? Is Annie (aged 3 in 1891) still with them and if so what is her name, relationship and birthplace?
Have you found your Ann b 1871 in the 1891 census? That is, was she still alive in 1891 (if she was, it would seem a bit strange for George and Isabella to name another daughter Ann, but if not it would make sense). Where was Ann's daughter Elizabeth, b 1885, in the 1891 census?
If your Ann b 1871 was still alive in 1891, then obviously a possibility is that Annie aged 3 in the 1891 census might be a grandchild (legitimate or otherwise) of George and Isabella. And it could be that she was registered under her birth father's name but listed as SCOTT in the census - which would explain why you can't find her birth. Another possibility might be if she was born in England.
You have indicated elsewhere that Elizabeth/Betsy would still have been alive in 1891 - have you found her in the census? And what about Helen/Nellie and Mary Catherine? Did they survive? Have you found them in the 1891 census? If Annie (3 in the 1891 census) is not with George and Isabella in 1901, is she by any chance with any of Nellie, Annie, Mary or Betsy?
There are probably all sorts of possibilities but more information is needed first.
Cheers,
JAP