Zoom H4N digital recorder issue

Hi I recently recorded some audio for some video work for a client's video that's going to be on their website. They're a huge client and this is my first video job since starting. Ive got a background in motion graphics but said I wouldnt mind helping them do some video work. So I came up with the storyboards etc and in the end it was me running 2 camera's (canon 5ds, one which I used with a shoulder rig) and 2 recording devices. One was a Blue Yeti hooked up to my microphone just to record the room tone and the other was a tie mic hooked up to a Zoom H4N digital recorder.

When I played back all the tests on the H4N the quality was perfect. All the audio was being saved into Folder 1, ready for when I exported it. After each interview I listened back and the quality was amazing. However, when I got back to the studio I imported the audio via the USB method and the files were super quiet. I dragged them into Final Cut and whacked the audio up to +30DB and you could then hear it well(ish). The downside is it sounds terrible. I contacted the company we hired the gear from and they said what I heard through the Zoom H4N when I played back the recordings should have matched up the audio when I dragged them off in finder (im using a mac). I really dont know what happened but now after half a days worth of shooting Im in the position where I have decent footage but literally crappy audio. Gutted isnt the word. Anyone help me or guide me on what could have happened and if I should prepare to be sacked?