Mac help -- migration

  I've got a new iMac, and a somewhat older iMac, and I want to migrate the older to the newer. I follow the directions with Migration Assistant, and while the new one usually recognizes the old one is there, the old one never shows the number that is supposed to be shown to allow the actual transfer to be made. I've tried it with WiFi and with Ethernet, and neither one has worked. Are there particular tricks for the migration I'm likely to be missing?