find my iphone saying offline but it is online

hi

my son has an ipod touch 5th gen

it is connected to wifi. when i use FIND MY PHONE using my own iphone it usually shows me he is online and his location.

his settings are correct and are locked as i have used Restrictions switched on on his ipod so that he cannot mess about with them (he is only 8 years old).

i know the ipod is switched on and online as i can imessage him without any problems.

when i try and use FIND MY IPHONE app, and log in, it tries to locate and then come up with message saying it is offline. it is not offline.

the device is only a few months old.

this function was working days agao. but now doesn;t appear to be.

he is about to go opn easter vacation and i wanted to keep in touch with him. obviously i can only do this when he is connected to wifi, but without the correct functionality of this app, i don't know IF he is connect NOR where he is