I really liked your work, I loved it!
The standard drawback of similar puzzles is that you didn't get a row to put together, so you have to make it work out somehow.
Here, on the other hand, you can try to put 12 together, and if that doesn't work, you can go all the way up to 20. Also, the different combining options are very helpful. It's a bit like Sudoku after all, although the purpose of combining elements is different.
It would be very curious if the game outputs total numbers. For example, there are several clusters of dice. One is shaped like a square, the other is a line. And both are missing one dice.
On pointing, it could show the already existing sum (or sum with dice in hand) on the line (at the edge of the grid) and on the square (in the corner of the grid).
However, that way it makes me to count the sum myself, which is useful anyway :D