optimization problem - solver falling to get correct answer

Hey,

I have a cell (D21) with a value that, connected by some simple calculations in other cells, is dependant on three, variable cells (not containing formulas).

When I keep the two of the three variables constant and run the solver in order to optimize the value of D21, the solver fails to find the correct solution (but finds a solution though).
I have a hard time to see why it fails to find the correct solution, and it is highly appreciated if someone can help me with getting around this problem in some way.

Please take a look at the attachment, which shows the problem more clearly.

Thanks in advance for your help