How do you know it's being used, and why are you checking it? Besides Noone here has access to your account. Call customer service 8009220204.
Is it an iPhone or smartphone, and the owner is now using an app that uses DATA rather than TEXT to exchange messages?
Another possibility - if the owner of the phone is traveling and in an area that is considered extended network or roaming, then the data may not show up immediately, but could be delayed several days.