autofs problem

I have autofs set up on my MacBook air running OS Sierra to automount an smb share from another mac on my home network. I followed the instructions here: http://blog.grapii.com/2015/06/keep-network-drives-mounted-on-mac-os-x-using-aut ofs/ . The mount point is NOT /Volumes, but /mnt/NAS, as advised in the instructions. The share is mounted upon restarting the MacBook air. However, after some time, which may involve moving the laptop out of the house, and returning, or the laptop being closed overnight, etc., I notice the share is no longer mounted. Running "sudo automount -vc" gives a message like "automount: /mnt/NAS updated" and no error messages, but the share is still not mounted. Rebooting the laptop restores the share. Any suggestions for troubleshooting?