There may be a pattern of which of the four beaches you can get this item from clamming based on time of day. I noticed at certain times of the day the east beach had plenty, and other times the spot right next to the NPC had them.
I did 5-10 buckets' worth of nonsense before realizing that the clamming point near the NPC on the North beach (dock) is not useful to get these. After switching to the East beach, I was getting 2-4 Jacknife within a few clamming attempts. Definitely recommend going over there, it's very fast.
