The OCR that Ancestry have used to transcribe the indexes is not perfect, largely due to the condition of the source index sheets that have been used.
I've found many of my relatives, who should be in there, at first glance aren't. That's because the OCR has misread one of the letters in the name (typically the mother's maiden name). For example, Harris is transcribed as Barris, Garrard as Carrard, De Gaetano as De Gaetans, etc.
So it's likely that your record is there, but mistranscribed. For the one you're missing, try searching without the surname and with the mother's maiden name, or with the forename, district and quarter, etc. You'll probably find it.