We suggest our users use at least a class 10 8G TF card and this kind of TF card will boot a system quickly and runs fluently. When using a lower class TF card it can work but its performance and user experience will be very bad. Some lower class card's read/write is only about 1MB/s which causes trouble in system booting and usually a class 10 card's is 10MB/s
Some users tried Kingston cards and didn't succeed. Sorry we haven't found the root cause for this issue yet.
We suggest you try the cards listed on our wiki:
http://wiki.friendlyarm.com/wiki/index.php/NanoPi_2