While some couldn't collect their winnings because they lost their tickets, "most of them failed to check the lottery results carefully enough,” the source said.
The people only checked to see if they had won the top prizes for two and three digits, while ignoring less popular categories, including the fifth prize worth B10,000.
Cursing their luck when it came to big prizes, "they immediately threw away their tickets or just left them on tables,” the source said.
By law, unclaimed prizes must be collected within two years of the lottery draw, said GLO chief Pol Col Bunsong Chanthrisi.
He advised punters to check their tickets against the winning numbers carefully or risk losing out on their right to prizes.
A sharp increase in the amount of unclaimed prize money has swelled state coffers to the tune of billions of baht since last year, the source revealed.
By comparison, between 2012 and 2017, unclaimed prizes totalled only about B650-800 million per year.
The government has also raked in higher revenue from lottery sales since 2016, when ticket numbers were increased to B100mn for the bi-weekly draw.
This year's revenue has hit B41.9bn, the source said.