Harvesting will stop if node you are connected will restart.
Also, if you using Trezor or you importing wlt file each time then wallet don’t remember which node you selected to harvest on. In such a case you must point to the node you were connected to allow check wallet on a correct node if you still harvesting there.
There is also a helpful tool which restarts automatically for you: http://nem-tools.com/
And tutorial for it: