password prompt for Exchange/Active on iOS devices

We are rolling out ActiveSync/Exchange eMail for iOS devices via MDM solution. Exchange accounts are created via MDM profile. Users are prompted with "incorrect password" prompt initially. After entering correct AD passwords, user wait for connectivity to their Exchange mailbox, and after 15-20 mins wait folder structure/eMail are downloaded on devices. Initial download completes.

Issue is starting where users are getting prompted for password on devices with either of the following on occasions "Password required "or "Incorrect password" and user has to enter AD password again.Sometimes the prompt take the password, and when it doesn't, user are directed to enter password on the Exchange account created on the device.

Two questions, first, why users are prompted for password prompts, example, "password required" or "incorrect password "when user password has not changed in AD. What settings on iOS device triggers this behavior. Second, why the password prompts don't take passwords.