If you are getting exactly the same results, then there are a few possibilities:
- you might have a mistake in your equation implementation that causes it to return the same values all of the time;
- you might have a mistake in how you specify the solver, causing it to always be set the same;
- you might have specified options such as integer constraints that are leading Problem Based Solver to override your solver request because the other solvers cannot handle the options / request
If you are getting the same results to within round-off error but not bit-for-bit identical:
- you might be using a simple linear equation or quadratic equation; all of the functions should converge for those kinds of function
- the result you are getting might be right at one of the constraint boundaries, so all of the solvers might end up at much the same solution
