I'm having trouble getting Keap to sync with Microsoft 365 email

Can anyone help with this query - I have successfully added my email account to Keap and emails are syncing perfectly (in the main!) However my colleague who has his own Keap account, has tried adding his account and it refuses to sync which means we are only getting 50% of our emails added to our contacts.
We have tried to add his account several times and each time we get the same issue - it sits as “pending” for a few days, then the message comes up that it “failed to Synchrnoise”