you have not added enough sodium hydroxide to precipitate all the copper as copper hydroxide. How could you tell that not all the copper had precipitated?

Answer 1

Two simple ways:
1. The blue or green color of the original solution is still there or
2. Let the Cu(OH)2 ppt settle and add 1-2 drops to the supernatant liquid at the top of the container and see if more ppt forms.