When 21 is in a grey circle, you know the digits 1-6 must all be used once for the sum.

When 21 is between the edge sum and sandwich sudokus it means that 2, 3, 4 and 5 are sandwiched and 1 and 6 must be used for the edge sum. Which numbers must be used for the 20 clue? Keep in mind that digits used in the edge sum cannot be sandwiched in that row, and vice versa.

Between the sandwich and next-to-6 sudokus a 6 can never be used for the sum. So that means when the outside clues sum to 15, the digits 1-5 must all be used once for the sum and the 1 must be next to the 6 in the next-to-6. Which numbers must be used for the 13 clue? In the row with the 8 clue, can two digits be placed next to the 6?

I hope this helps :)