Why would cymbalta make a difference than the anti depressant pristiq (desvenlafaxine) pleaseb?

Different people: People have different responses. This is true for all medicines. Some medicines work better for some people than for others. It's not entirely trial and error, but it can be surprising how differently people respond to medicines. Changing one medicine to another- the goal is that it would be more effective and cause fewer side effects.

If you are not comfortable or unsure about the medication your Doctor prescribed then it is advisable to get a second opinion from a Psychiatrist after you have had a complete physical exam by your Primary Care Physician to rule out any underlying illness. A Clinical Psychologist can also help you with psychotherapy.