Help needed with transportation problem using MODI method

How's it going everyone?

I'm doing a transportation problem using the MODI method. I was required to set up the initial table using the North West Corner method and then solve the problem with MODI. After 6 iterations I have 3 positive Opportunity Costs and 3 negative. The 2 smaller (absolute) negative OCs are associated with dummy cells. If I use the largest (absolute) negative OC I end up with a table identical to a previous iteration. If I ignore the largest (absolute) negative OC and go with the next lower (absolute) negative OC I end up after 11 iterations back at the initial table. Does this mean that the present iteration (the 6th) is the optimal solution, or have I just made a complete mess of things? Any help would be greatly appreciated, thanks.