Can I enter a loan after the fact?

" I just noticed that the Principal/interest split has not changed since the start."

Where are you seeing this?  In the check register, in the loan's register, or both? 

Is this a "downloading" loan where the lender is providing the principal balance adjustments to the Quicken loan Account?

Assuming for the moment that the check register and the loan register are consistent - both showing the same amount of principal reduction with each payment - then if you want to change these entries after the fact to he correct amounts you will have to do this manually.  Having done that then you'll want to set up a new loan Account (correctly) for the loan such that normal amortization can occur. 

Assuming you set up the loan using the original origination date, amount, etc. Quicken will make two entries in the new loan register - an amount as of the origination date in the opening balance of the loan and then an adjustment amount as of the day you create the new loan to bring it to its current balance.  You can then choose to either close out your first loan Account, or delete the two opening balances in the new loan Account and transfer the balance of the old loan Account to the new loan Account as of the day of creation, leaving the old loan Account zeroed out.