Repeated Microsoft 365 sign-in trouble is frustrating precisely because each incident gets fixed and then returns. A password is reset, a session is cleared, and a few weeks later the same complaint arrives. That pattern usually means the underlying cause has not been found. These are the most common ones.
1. Multi-factor authentication that is only half set up
MFA is now standard, but it is often rolled out unevenly: some users enrolled, some exempt, old authentication methods still enabled, phone numbers out of date. When someone changes phone, or the method they enrolled is retired, they are locked out until someone intervenes. A consistent MFA policy, with a clear process for replacing a lost device, removes a large share of repeat lock-outs.
2. Conditional access rules that nobody remembers writing
Rules that block sign-ins from certain locations, devices or apps can be sensible, but they accumulate. A rule added for one incident may quietly block travelling staff, a new office or a legitimate application months later. If the same user pattern keeps failing, review the sign-in logs for the reason a sign-in was blocked rather than resetting the password again.
3. Old sign-in methods and legacy protocols
Some older email clients and applications use methods that modern security settings block. The result looks like random failures or repeated password prompts. Identifying which applications still use those methods, and moving them to modern authentication, fixes the cause instead of the symptom.
4. Devices in a confused state
Devices that were joined, unjoined and rejoined, or that carry stale cached credentials from an old account, can prompt repeatedly for sign-in. Cleaning up device registrations and standardising how devices are enrolled often ends "it keeps asking me to sign in" complaints for good.
5. Password and account hygiene
Expired passwords, accounts shared between people, and accounts with unusual sign-in activity can all produce lock-outs. Repeated lock-outs on one account can also be a sign of someone else attempting to sign in, so it is worth looking at the sign-in logs before assuming the user is simply forgetful.
6. Licences and administration
Licences that lapse, unassigned services or several administrators making overlapping changes can cause access problems that look like sign-in faults. Knowing who your administrators are, and keeping their number small, makes changes easier to trace.
How to narrow it down
Ask three questions. Is it one user or many? Did it start after a change, such as a new policy, a new device or a licence change? And what does the sign-in log say about why the attempt failed? Those answers usually point to one of the categories above.
When to get a second pair of eyes
If the same problems have returned more than once, or you are not sure who owns the Microsoft 365 configuration, an independent review is quicker than another round of fixes. A Microsoft 365 Security Review checks MFA coverage, administrator roles, conditional access, legacy sign-in, sharing and licensing, and gives you a written, prioritised list. If something is broken right now, Microsoft 365 Support can help diagnose it remotely.
These services are advisory and support work. They do not guarantee that no incident will ever happen.