I have some experience with Sense and dryers. Here is a snip from my Sense records:
It found my 240 volt, electric dryer just 12 days after Sense installation, which lends support to your idea that it is easy to detect. A month later it detected half the device separately. I replaced the physical device a month after that, so deleted my Sense definition. Sense then found its ghost as a new device as described in this post:
The new dryer was also a 240 volt, electric dryer. It was not discovered quickly, so large loads must not always be easy to detect. The new dryer was discovered in stages. First it found a portion called Dryer 3. That portion was exactly half of the heating-element-only device called Dryer High, so I deleted it. Later it found half the heating element again, but this time together with the motor (which must be 120 volt).
During this time, my Dryer High continued to function, so the heating element on that leg was double-counted. Although the double counting was a little disconcerting, I put up with it. That turned out to be the right decision, since a month later Sense combined the two devices into a single device. The new device measures both legs of the heating element plus the motor on just one leg. I am very happy with the current detection, although it sometimes misses the motor-off signal.
In conclusion, detection of half the power does not seem unusual. Since you have already tried being patient, the other option is to delete the device and hope for re-detection. Since you say Sense does not always see even the half it has, deletion is a good option. I had a window AC unit that was only finding 30% of usage, and I was too patient with it. When I finally deleted it, I got a new detection that was much better.