My beginning statement balance doesn't match last months ending balance.

The "Beginning statement balance" shown in the reconcile window is the cleared balance on the account. Which is to say it is the sum of all the transactions in the account that have been marked as cleared (plus the initial balance on the account if you had one).

If you reconciled your account last month and this month you came to reconcile and the beginning statement balance this month does not match the ending statement balance from last month there are a limited number of possible causes.

1) You modified the amount of a transaction that was already marked as cleared.
2) You deleted a transaction that was already marked as cleared.
3) You manually cleared a transaction in the intervening time.
4) You deleted a category that contained cleared transactions. Deleting a category deletes all transaction that use that category.
5) You deleted an account that had transfers to or from the account you are reconciling. Deleting an account will delete all transfers that use that account.
6) You changed the "Initial balance" on the account under Account->Edit Account...
7) You manually marked a cleared transaction as uncleared.
8) If you had the option under Preferences->Network "Mark transactions as cleared when confirmed" checked and then performed a download and confirmed transactions.

If you have done any of these things it can be quite tricky to track down the cause of the change in cleared balance.

The only way to fix such a problem is to go over the register and your old back statements and see if you can find anything that has changed.

If the discrepancy is relatively small it might be simpler to enter a new transaction (Categorize it "reconcile discrepancy") and clear that transaction.

Some users find it helpful to enter a dummy transaction with a zero amount and enter a description stating what the cleared balance was on this date as a marker they can refer back to later.