There are a large number of possible causes. Could you please provide more details?
0) What firmware, GCS software, and hardware versions are you using?
1) What things are connected to the PixHawk? How are they connected? (Pictures are great, if they're clean)
2) What steps do you take to produce the problem?
3) What tests have you done to show that the issue is with the PixHawk? (Not a faulty servo, bad TX/RX, etc)