I noticed this as well. I'm thinking that perhaps the planes are sitting on the edge of the range for the stock antenna. If this were the case, then adsb# may only be decoding what it can, which is why you get some planes with no data beyond the ICA024 and Registration, while others you might get the altitude or heading, etc. The signal may be too weak, or there may be too much interference for the receiver to pick up the full packet of information.