Smartthings as a platform does have issues with marking devices “off-line“ which are not actually off-line. That can be annoying and hard to fix. But before we get to that, let’s first look at some of the reasons why, regardless of the Home Automation platform you are using, a Zigbee device like your sensors might have trouble communicating to the hub. That is, situations where they are truly off-line.
First, Zigbee as a protocol is designed for “mesh“ communications. In order to get good battery life, the devices are designed intentionally as low power transmitters. Rather than having each sensor communicate directly with the hub, in most cases, a battery powered device will be communicating with a mains powered device nearby. that second device then Sends a message a little further down the line to another mains powered device. This is called “repeating”, and it’s sort of a pony express setup. Low power device sends the message to the limit of its range, which for Zigbee home automation is only about 40 feet. Then the next repeating device picks up the message and sends it along for another “hop“, another 30 to 40 feet. And So on, until eventually the message gets to the hub.
Many people who are unaware of this process will buy a bunch of batterypowered Zigbee sensors, Place them all over the house, but not get any mains powered Zigbee devices. They then find that only the few sensors that are within one hop of the hub can get their messages through, because battery powered devices generally do not act as repeaters.
So first, let’s consider the layout of your Zigbee mesh.
Begin by reading post 11 in the following thread, then after you have read that, go up to the top of the thread and read the rest of the messages. That will give you the basic concepts and terminology You need to build a strong mesh.
I will link directly to post 11:
A Guide to Wireless Range & Repeaters - #11 by JDRoberts
After you have read that, if you think you do have the right devices in the right places to create a strong mesh, we can start looking into the questions that @jkp just asked. 