Issue can occur when you try to pair two Phantoms and the firmware version of the units isn't the same.

The units are both found by Spark, then when you try to pair it will tell you that one unit needs updating. When the update runs there is no way to see the progress or when the unit has been updated. 

Upon the update being done, you need to run Spark again and redo the full setup for the pair. 

It may take over 5min for the unit that is being updated to show up again in Spark. With no way to see that it's ready, you just have to keep closing/opening Spark