fall off

If the amount, rate, or quality of something falls off, it becomes smaller or lower:
Sales have been falling off recently.
Membership of the club has fallen off in recent months.

a reduction in the amount, rate, or quality of something:
A sharp fall-off in US and European demand for Chinese exports would have a big effect on Hong Kong companies.
We have seen seen a dramatic falloff of demand in the final quarter.
(Cambridge Advanced Learner's Dictionary & Thesaurus からの fall off の定義 © Cambridge University Press)

アメリカ英語辞典
to become less in number, amount, or quality:
Production fell off last month.
(Cambridge Academic Content Dictionary からの fall off の定義 © Cambridge University Press)

ビジネス英語
to get lower in amount or level:
Orders have definitely fallen off in the past quarter.
(Cambridge ビジネス英語辞典 からの fall off の定義 © Cambridge University Press)

- Cambridge Dictionary