So if you were seeing the X TFC symbol and not the X Fail symbol, then that means that the TIS system has stopped tracking you (you're not logged into it). Know that the TIS system is only at some class B/C/D airports, and the coverage is gradually shrinking as those airports get upgraded to the latest radar. So if you were at the edge of a coverage range for a particular airport, then this is what you might see at times. It could also be a glitch in the TIS system at that airport. Finally, if there was an issue with your transponder antenna setup, you might see intermittent TIS coverage as that ground station lost you. But you'd also expect more generic transponder reception issues with ATC as well.