The un-harvestable nodes are usually a result of poor harvesting. If someone does not take everything on the node - it will become bugged.
Be a considerate harvester: Take everything from the node - do not leave anything behind.
As to the spawn points for nodes, for the most part, they are hidden behind things such as walls, rocks, tents, rubble, etc. They can be poorly placed too. They can be IN the wall, rock, rubble, etc. and inaccessable. These are not the nodes your looking for - move along. (Has this joke been done to death or what!? )