There are multiple possibilities why a transaction is not matching:

1.  The transaction does not yet exist in R365, but can be created via bank activity.
2.  The transaction it should match to is in a closed period (see Legal Entity record).
3.  The transaction in the system is not the same amount as what cleared the bank.
4.  It isn't matching undeposited funds because you do not have the boxed checked 'Match Undeposited Funds'.
5.  The date of the transaction in R365 is more than 2 days after the date it cleared the bank