Ammonia forms the complex ion[Cu(NH₃)₄]²⁺ with copper ions in alkaline solutions but not in acidic solutions. The reason is 

Copper hydroxide is an amphoteric substance.

In acidic solutions hydration protects copper ions

In alkaline solutions insoluble Cu(OH)₂ is precipitated which is soluble in excess of any alkali

In acidic solutions protons coordinate with ammonia molecules forming ${\mathrm{NH}}_4^{+}$ ions and NH₃ molecules are not available