I just learned (from a Support reply) that the Sense monitor may go off-line if the WiFi signal is too strong!!
The recommended range is -30 to -70 (dbm); the support staff looked at issues I was having and reported that my WiFi signal strength was often -27 … and that the WiFi chip can get over-saturated and take the monitor off-line.
I ran the Sense network test myself (twice) and got -31 and -33 … close to the maximum recommended. I then moved a metal plate to partially cover (~ 1/4 of end of) the antenna, resulting in a -40 reading via the built-in network test.
Hopefully that will cure the drop-outs I was having from a too-strong signal.
I also note that this issue (too strong WiFi signal) is not noted in the list of possibilities in this topic: