Randomly Placing Prefabs on a Grid

Hello,

I want to randomly place prefabs at specific locations (think a 5x3 grid) at random times (ranging from 5 to 15 seconds). However, the items should only be allowed to be place at a grid point if there isn't already an item there.

The way I have it coded now uses an array based on grid location that keeps track of if an item is currently in that spot (an array of booleans).

I have a set of if statements that check if THAT grid spot was chosen AND if there isn't already an item there. But, it seems to fail because when I test items get placed multiple times at the same grid location.

For example:

 BoardLocation = Random.Range (1, 16);
             if (BoardLocation == 1 && SpotTaken [0] == false) {
                 XYLocation.Set (0, 12, 0);
                 SpotTaken [0] = true;
                 return XYLocation;
             }

Any ideas why this isn't working?

Thanks