Are you on the latest firmware version? If not, try to update your Sylphyo first. We fixed some bugs with touch sensing since the earlier versions.
What fingering do you use? What could happen is that sometimes your left index finger, or your left thumb, or any other upper finger slowly drifts from its original position, and the touch sensor considers that the finger isn't touching the key anymore. The resulting fingering is sometimes not "understood" (because without the left index key it doesn't correspond to an actual recorder fingering anymore, for instance), and in this case the Sylphyo just stays on the current note.
If you see this happening and you want to correct it, just release all keys for a second and place your fingers back in the desired position. To avoid this situation in the first place, you might want to make sure the fingers that touch the keys are placed in the center of the keys, and that you touch new keys by hitting them a bit more frankly (as the touch sensor might have trouble when trying to recognize a slowly approaching finger).
