How long have you had these birds, if you havent had them long you may have let them out to early and they dont know where the loft is, this sounds silly but i have seen it time and time again bring in new birds let them out straight away, and never seen again.
if this is the case you will need for them to notice the other 7 birds just tottering around, what i would do is leave the other birds to go hungry and let 6 of them out when the other 3 birds are around and the other 1 hold it and throw it towards the other 6 to try and get the attention of the others also keep make the other 6 flutter around as young birds are like sheep they just tend to follow each other and if they see the others fluttering around they may come down..