Need some help solving the attached diagonal puzzle. It is listed as #15 in the hard starters section of the Sudoku Library app. I've put it through "Sudoku Solver", however it is not able to solve the puzzle. Have I missed entering a number somewhere?

your small hint numbers are correct; you can see that there is only one cell in row 7 where you can fill in the "3" which is in column 8! After that, you can place another "3" in row 4, column 4 when taking the diagonal constraints into consideration as well.