By far, the most common cause of temporary data dropouts between a sensor and a gateway (e.g. the last reading being more than a minute or two old) is excessive distance and/or obstructions between the components.
If you're having issues with sensors coming and going from your gateway, check the signal strength as reported by the gateway. You can see this in the Summary section below the graphs on the sensor detail screen (tap the sensor in the main list to access.)
This sensor was last seen 39 seconds ago by "New Gateway 8" with a signal strength of -72dB (or 2 dots.) This sensor is on a different floor than the gateway, but we still have a good signal strength, and this sensor works consistently with this gateway.
However, as the reported signal strength approaches or dips below -85dB which is the threshold for one dot of signal strength, the possibility of intermittent connections increases. Notice that the numbers are negative, so -85dB represents a weaker signal than -72dB.
The usable minimum depends on the specific environment, but the key point is that if your sensor isn't reporting consistently and the reported signal strength when it does report is less than -85dB or so (or, more simply, you only see one dot for the signal strength), you can likely fix the problem by either relocating the equipment to reduce distance and/or obstructions, or even just reorienting the antennae. The sensor antenna is internal to the device, so sometimes just reorienting the sensor can help.
The signal strength has to be passed from the gateway to the cloud and onward to your app, so it can take a couple minutes for this to update, so you'll want to change something and give it a bit of time to fully see the effect.
Also keep in mind that water, and as a result the human body, is a very effective absorber of Bluetooth signals, so if you are standing between the sensor and gateway (and won't normally be when the system is operating) this will skew the results, potentially very significantly.
If you're unsure, one thing that we often try just as a test, is to temporarily place the sensor next to the gateway and see if the issue goes away. If it does, then it's almost certain that we're facing a range issue and changing the physical arrangement of the gear will fix it.
If you have a range issue and are not able to address it by repositioning your equipment, you can add one or more additional SensorPush G1 WiFi Gateways to your system to help reinforce or expand the wireless coverage. Just add them to your app as you did the initial gateway. There's no need to specify anything beyond that. The gateways will all report any sensors they are able to see, and any overlaps are sorted out automatically.
For the purposes of informing placement of equipment, if you have multiple gateways and a sensor has recently been reported by two or more of them, you'll just see additional rows in the signal strength message above that give the names of each gateway as well as the time since it reported the sensor and the signal strength.
Learn more about signal strength and SensorPush in these articles:
Signal Range and SensorPush Products
What is the wireless range of SensorPush sensors? How far will the signal travel?