While roasting a duck, the skin gets crispy when there is no fat below the skin. Thus, you need to make sure that the fat below the skin melts as quickly as possible and afterward can flow somewhere else. To let the fat flow away, pierce the skin with a fork (or tip of a knife). To melt the fat even before you put the duck into the oven, pour it with boiling water.
While the duck is being roasted, check whether the fat really flows away. Poke the skin with a fork. If you find any "soft" areas below the skin, this is probably where the fat is and does not have a chance to flow out. Pierce those areas.