Why can’t I compound my staking interest on Stakeboard?

You can’t automatically add your interest to the principal amount staked as Stakeboard is not a wallet; it is software that allows you to interact with the KILT blockchain. Therefore, Stakeboard does not actually hold KILT Coins. Your rewards go back to the account you staked from and need to be restaked manually. See instructions for restaking in the “How do I increase my stake” question.