Ben,
Are you running FCP 6? If so, head up to the FCP Menu:
Modify/Audio/Apply Normalization Gain
Also, you may want to consider a loudness maximizer as opposed to basic Peak Normalization. Final Plug 5 is an AU Plugin that works really well.
https://www.wavearts.com/FinalPlug5.html
Simply decrease the Threshold setting, set a Ceiling [typically -0.1dB] and fire away. This will result in an increase in average loudness. Much more efficient. Use it sparingly though. Aggressive processing can potentially destroy the dynamic range of your segment.
-ptfigg.