When does a bill charge CGST and SGST instead of IGST?

It’s automatic, based on the customer’s state. Same state as your company → CGST + SGST split evenly. Different state → IGST on the whole amount. It reads the state on the party’s own record (or the one you type for a walk-in), so make sure that is correct — an unselected walk-in falls back to your own company’s state.