My sudoku board generating function is getting stuck in an infinite loop - c++

Question

The user is supposed to input a number between 0 and 81, and it should generate a sudoku board with that many (valid) numbers filled in (Even if the sudoku board may not be solvable).
It works fine for small numbers of n, but if it has to backtrack, it gets stuck oscillating between ~42 and ~56.
In this program, it's important to note that n is the number that the user has specified, nOn is the number the function is on.
the array named positions[] contains the numbers 0-80 in a randomized order (compliments of the shuffle function) and is designed to represent the randomized order in which to fill the grid.
bool Sudoku::generateSudoku(int n, int nOn)
{
int xpos = positions[nOn] / 9;
int ypos = positions[nOn] % 9;
int queryCount = 0;
char query = rand()%9 + '1';
bool success = false;
if (nOn >= n)
{
return true;
}
else
{
while (queryCount < 9 && !success)
{
if (isValid(xpos, ypos, query, 0))
{
sudoku[xpos][ypos][0] = query;
success = generateSudoku(n, nOn + 1);
}
if (query == '9')
{
query = '1';
}
else
{
query++;
}
queryCount++;
}
if (!success)
{
sudoku[xpos][ypos][0] = '*';
return false;
}
else
{
return true;
}
}
}
I've gone over it several times with 2 of my friends and I can't find why it's getting choked up. Any help would be appreciated!
edit: I've added a similar function that I know works for comparison. This one starts in the upper left hand corner and recursively backtracks when it runs into invalid solutions. it's able to solve any (solvable) puzzle in a matter of seconds
int Sudoku::solveSudoku(int x, int y)
{
bool success = false;
char query = '1';
int counter = 0;
if (x == 8 && y == 9)
{
return 1;
}
else if (y == 9)
{
y = 0;
x++;
}
if (sudoku[x][y][0] != '*')
{
sudoku[x][y][1] = sudoku[x][y][0];
success = solveSudoku(x, y + 1);
}
else
{
while (query <= '9' && !success)
{
if (isValid(x, y, query, 1))
{
sudoku[x][y][1] = query;
success = solveSudoku(x, y + 1);
}
else
{
query++;
}
}
}
if (success)
{
return 1;
}
else
{
sudoku[x][y][1] = sudoku[x][y][0];
return false;
}
}