Drawback of Waterfall Model
Each step in the Waterfall model is a discrete, standalone process that follows the one before it. If in a Waterfall model you reach the last stage and find that something should have happened further up, then it's too late to go back.
Also See:
Waterfall model, When to use, its advantages & disadvantages
Incremental model, When to use, its advantages & disadvantages
RAD model, When to use, its advantages & disadvantages
Spiral model, When to use, its advantages & disadvantages
Comparison of Software Development Models
Questions on Waterfall Model