DJM Rec app not recording

Hi, 

I did a session in a studio today, and recorded a series of mixes which are all displaying in terms of length in mins/secs but do not play with any sound and there's no waveform. I used an iPhone with Software version 14.4 and a Pioneer DJM MIXER 900NXS2 4 Channel mixer. I assume the latest firmware was on the mixer & have used the app before and it recorded fine. It's quite frustrating as I needed to hear the mixes back so appreciate your help. Thanks, C