Audio Mastering

Hello house, i need your advice i’m new here. I use cubase 5 for music production. I’ve been using the cubase mastering setup to master my beats and they come out loud and clear after exporting, but after mixing a song today with vocals i mastered the song and it was very loud inside cubase but when i export the sound drops and the audio is very poor. Please how do i export my mastered song and still get quality in terms of heaviness and loudness inside cubase. I can achieve this with instrumentals without voice.