Thanks for playing! One of two reasons - either they've exhausted the resources on the adjacent tiles to where you sent it, or you've run out of storage space for that resource.
On the first point, I didn't want the game to just set-and-forget your Harvesters and they just roam the entire map. As each block of 3-4 resource tiles is exhausted, you have to send them to the next one.
On the second point, Depots and Habitat modules will help you expand your resources.
