By "doesn't make a notification sound", is your phone still receiving messages silently, or is it not receiving any messages?
If it is the second case, have you checked your network signal strength in the warehouse? If you see no "bars" of signal, that would mean your phone cannot connect to the cellular towers, and therefore cannot receive messages. Some buildings are worse than others for radio signals, especially ones with metal structures.
Also, if your phone was working fine on Lollipop, you could attempt a downgrade from Marshmallow at your own risk, after backing everything up first. People using
this guide have found varying levels of success.
But you should first make sure the issue is indeed with the phone, and not the network.