Could be several different things
Look to see how OTM is setup to collect data
DBA? Serial Port?
if DBA check switch - LD 117 STAT Buf This will show if enabled and if connected. If enabled but not connected try terminating OTM and restarting - DBA should start automatically
If serial, Check your serial connections, Check switch - LD 37 stat
Connect port to lapto and see if you are getting data