Botswana has a trenching programme running at the Thorny River project in South Africa, and a 300-tonne sample from late 2017 was not as promising as expected, the company said.
Over 500 diamonds for a total of 62 carats were recovered from the sample, with a pricing estimate of US$89 per carat.
The company said there weren't enough diamonds in the sample to produce a "modelled diamond valuation" so it will stick with a value of $259/ct.
Botswana chairman John Teeling said the volume was not an issue.
"We have made rapid progress at Thorny River with the generation of volume and grade estimates following intensive exploration field work," he said.
"We recovered insufficient diamonds, particularly the bigger sizes, to produce a valuation which could be used in economic modelling. This will be addressed."
Botswana's shares were down around 10% on the news.