Standard solver in a macro
 

I am currently using a the standard solver add-in in a macro. Although the
procedure does yield results, I think that the optimisation (linear) is not
running correctly. The target cell is maximised by changing x. If I guess
an initial value for x as 10, the "maximum" of the target cell is a negative
number (and x changes to 15). If I stop the macro and manually change x to 5
the target cell is a positive number. The constraints are 0<x<20. I do not
understand that if the intial guess for x is 10 why the optimisation
algorithm will yield a maximised optimum of a negative value at x= 15, rather
than a positive value at x = 5? In the formula for my target cell I use
LOOKUP and IF statements. I read in the user guide for the Premium solver
platform that IF, CHOOSE and LOOKUP functions may cause difficulty for the
standard solver.

Is this the problem that I am experiencing and will installing and using the
Premium solver platform sort this out?

Thank you for your help in advance