If you're a restless sleeper, the Somnofy will have trouble determining your respiration rate.
Since Somnofy is based on radar technology that measures your movement, if you move around a lot in bed, Somnofy will not be able to register your breathing. This does not mean that you have not breathed while sleeping, just that you have moved a lot.
Unfortunately, there is not a lot we can do to fix this. However, the measurements can be improved by trying different positioning of the Somnofy, preferably quite close to the user.