In terms of Sense not detecting a specific, fairly common/popular water heater model, @brianmur had a pretty good summary of why Sense may detect it for some users but not others. To provide some more context: though the water heater described draws a lot of power and is on both channels, it is possible that a particular house is “noisy” (a lot of other devices turning on or off) or has a lot of 240V devices. Factors like that can make Sense’s job of creating models for certain devices easier or harder in different homes.
In regards to Sense forgetting devices: I replied to Long Running Devices - #3 by HilarioAtSense recently in regards to this. It really shouldn’t be happening so frequently, so please contact our support team so our data science team can take a look at the models for these devices.