What phone do you have?
What app are you trying to read with?
Try these
TagInfo
Tag Writer
Have you implanted the NExT? or are you trying to read whilst it is still in the syringe (You wont be able to)
If it is implanted:-
Make sure your Phone is unlocked with screen on.
Make sure NFC is turned on
Take any phone cover off the phone
Try reading NexT on the back of the phone ( Top edge for iPhone ) in different orientations and move slowly, Check this guide for KNOWN compatibility.
Give that a try, and let us know how you go