I see others have twigged as to why you are unable to find your ancestor in the Sheriff's Court Paternity Decree Indexes on Ancestry. It's because the indexes on Ancestry have come from ScottishIndexes, and not OldScottish, two companies who do pretty much the same thing in terms of photographing and indexing Sheriff's Court Paternity Decrees in Scotland, although their areas of coverage clearly vary.
Note that Ancestry only has the indexes, which is the same information that you would get by searching on the ScottishIndexes website itself, and which would be more up to date in any case. For the original documents, which will be photographs of the relevant pages of the Sheriff's Court registers, you would have to order them directly from OldScottish in your case. I don't know how much OldScottish charge, but I have ordered Sheriff's Court Paternity Decrees from ScottishIndexes, and have found the cost to be minimal, and the service excellent, the turn around time is within a couple of days. The record itself will probably cover just under a page of the register, and should have also led to an entry in the Register of Corrected Entries which should in turn be linked to the original entry in the Register of Births on ScotlandsPeople.