When a go-around is initiated with the aircraft in landing configuration (landing gear, flaps etc. extended), a loss of altitude is inevitable while the engine accelerates and the pitch change and increased power take effect.
However, if a go-around is initiated when airspeed is already below the prescribed figure because engine thrust has been and remains too low, then the approach may already be unstable. Recovery from such a situation necessitates careful handling as thrust is added and pitch attitude changed.