At an event that I attended recently, the conference pack included the usual feedback form. It was a bit smaller and printed on stronger paper, so it was more of a card than a sheet of paper. At the closing session of the event, one of the organisers reminded the attendees to leave feedback and put the feedback cards into the provided box. Said box stood on a nearby table. It was half-transparent and the sun happened to be shining on it at that moment, so you could clearly see that it only contained one feedback card at that point (which happened to be mine - I had just filled it out before the closing session). The woman was clearly shocked and had to ask her team if someone had emptied the box before (answer: no).
While having only one feedback form at the end of a full-day conference is indeed a bit on the low end, an overall low return rate at that point shouldn't really come as a surprise; it's a simple matter of logistics.
