I don't know about the flight times stated by OP, as they seem to be incorrect. But, currently, 9W operates 9W60: BOM 1235 - 1825 BKK.
This flight isn't available for booking in August.
IIRC, 9W was 3x daily on BOM-BKK-BOM sectors. But recently they have started codeshare with PG. That along with the off-peak season may be the reason for reduced frequency.
Contact 9W and modify to alternative flights with lower transit time.