Hey @deathweasel—sorry to hear the setup on the 2nd one isn’t as seamless—check out the troubleshooting guide for a clear list of steps to go through:
Most strange is the fact that the Core continues blinking blue, never turning solid blue then green to indicate it has heard the Smart Config signals. Connecting the Core via USB isn’t too bad—instructions here:
http://docs.spark.io/#/connect/connecting-your-core-connect-over-usb
If you’re comfortable using a terminal on your mac, here’s some more detail. Try connecting your Core to your mac with the USB cable, then do a factory reset. After it settles into blinking blue, unplug and replug the USB cable from your computer. When it again settles on blinking blue, in Terminal run sudo ls /dev/tty.u*. This should list at least one file with a name something like /dev/tty.usbmodem621.
You can run sudo screen /dev/tty.usbmodem621 (replacing the filename with the one recognized on your computer) to open a serial connection. Hit i to have your Core print its ID. Hitting w will let you enter Wi-Fi credentials. Factory firmware can only handle WPA2 networks over USB, but once your Core connects and updates, more security modes are available.
If you go through these steps and are still having trouble, go ahead and email me at zachary@spark.io. Cheers!