Welcome to the frustrating world of Freeview. I have also experienced this, it simply means that when you scanned for available frequencies and channels, your set could not find, on these occasions, what it found before. I have an indoor loft aerial, not ideal I know, but when reception is poor, I have sometimes only picked up about 20 channels, whereas at times of good reception I can regularly receive 88. Only answer is to rescan at another time, or upgrade your aerial/distribution system.