Hi and a happy new year!

I still got problems with an production optimization. In the attached excel
file you will find demand data, maximum and minimum stocks, production
capacity and stocks for 13 months
I need to satisfy demand each month by the use of stocks, new production or
purchase. The use of stocks is the cheapest alternative and purchase the most
expensive alternative. The sum of provided quantities times costs should be
minimzed.
My problem is the following:
The production should happen in batches of at least 500 pieces (if capacity
is sufficient) or should be zero if stocks are sufficient to meet demand.
This is why I introduced a binary variable which is linked with production
quantities.
When I solve the optimization problem with the Excel Solver, the binary
variable and the production quantities in each month are zero but the more
expensive purchase is always positive.
Does somebody know why? Are there constraints wrong or missing?

http://www.herber.de/bbs/user/66969.xls