This is almost always related to cellular service. It can be frustrating as you may see bars on your phone, voice works OK, and it may have worked OK on a different day, but that does not necessarily mean that data is steaming well enough where you are at that time. One easy check is to open a browser on your field data collector and navigate to a website (choose one that changes a lot, as you may see a cached page otherwise). Another tool is a cell status app, like Network Cell Info Lite, that shows the signal strength in detail (for instance, a value of -100 dBm – 110 dBm often means spotty data streaming, and -110(+) dBm can mean no data streaming). Because voice has priority over data, even though you show bars and have voice service, the data streaming may be spotty or not flowing. This can be due to high voice traffic at the time and area you are located and it can pass just as fast as it happens. iPhone users can also view a native analysis tool by cutting/pasting or typing *3001#12345#* into the phone dialer; this will show much the same info as the abovementioned app.

WHAT CAN I DO IN A POOR CELL ENVIRONMENT?