Remember that NH3 is alkaline/base which produces hydroxide ion in water,
NH3 + H2O NH4(+) + OH(-)
Limiting NH3 will give you precipitate of copper (II) hydroxide as this reaction occur:
CuSO4(aq) + 2NH3(aq) + 2H2O(l) -> Cu(OH)2(s) + (NH4)2SO4(aq)
But in excess of NH3, soluble complex compound will be formed which makes precipitate disappear
CuSO4(aq) + 4NH3(aq) -> [Cu(NH3)4]SO4(aq)
I hope this explain everything.