It's only saving the last 30sec of time, not active conversations. So if there was a conversation and it happened 30 sec ago then there will be no audio to be played back. Set the time to 240 sec. The normal life span of a SD card will let it record something like 10 years of continuous audio, if you set the squelch to 0 and let it record noise 24/7/12/365.
Manual recordings, that are left on the SD card and not deleted after 240sec, will fill a 32GB SD card after a couple of weeks and will then display a warning that the card is full and awaits a user response until it can continue to scan, if I remember it correctly. It could also react differently depending of model or firmware version, that it just warns that recording have stopped and then continuous to scan.
/Ubbe