When using refresh token api, sometimes box replies "Bad Request(400)"

We use Box's API to upload files to Box folder regularly.
But sometimes refresh token fails with 400 (Bad Request)
(This occurs very rare, but, If this happens, the customer's re-certify is needed.)
How can we avoid this?

--- our using API is following.---
https://www.box.com/api/oauth2/token?grant_type=refresh_token&refresh_token=%RefreshToken%&client_id=%Client_Id%&client_secret=%Client_Secret%