Two simple rules apply.
- If it was easy, then all solvers would do it for you, automatically. There would never be any issues. No complaints. No failures from this.
- See rule 1.
Essentially, this reduces to finding SOME solution, ANY solution to a set of equality and inequality constraints. Sometimes that really is easy. For example, if you have only linear constraints and bounds, then finding a solution is relatively trivial, at least in terms of the linear algebra.
However, if the constraints are nonlinear, then things get nasty. At least, they can do so.
In fact of course, fmincon DOES try to find an initial feasible point. Does it always succeed? Of course not.
Now, take a look on Answers. Count the number of question that were asked where people have posed a problem for the solution of some system of equations, but a solver fails to find any solution. It is actually not that uncommon. Sometimes it means the person has posed an impossible problem with no solution, yet does not realize it. Sometimes a solution may exist, but the solver could not find one. And sometimes, it is the fault of the person writing the code. They made a mistake, and formlated their problem incorrectly.
Matt J said it very well, that we cannot give you more help beyond the generic without more information.
