This is most likely a false offline detection. You need to change the SmartServer channel settings to reduce or eliminate this false offline detection.
The SmartServer default configuration is to report a Device, FB or NV as offline when a single response packet, due to sending a request packet to the device, is lost or the response takes too much time to get back to the SmartServer. This will cause the Device, FB or NV/CP to show up as offline in the SmartServer Web Pages. Thus the SmartServer default settings may falsely indicate that the device is offline.
To reduce the false offline detection go to the Go to the Channel (Driver view) in the SmartServer Tree and look at the Offline and transmit (advanced) settings. You may want to modify these to eliminate the false reporting. Click the Help Menu to get more information about each setting.
You should try making the offline reporting time 2-4 times the poll rate. If you polling every 15 minutes then change the "use Minimal Offline time" to 60 minutes