Len's telling you the straight truth.
Your dealer does need a good bit of educating. This sport is chock full of oddball ideas and urban legends, but you hate to see it come from behind the counter of a bow shop.
RI, if your draw was closer to 30", you'd be a lot more likely to see that arrow nock falling off the string at full draw. Even at 27", I bet you'd soil your pants if you saw the angle the string is taking inside that nock when you're at full draw. Probably only the top corner of the nock is still on the string.