You may also be missing the fast forward frames after the program completes, if a problem occurs with during the recording. This has only happened once for me, when I was having OTA signal issues. When this happened, My recordings were also being split into multiple parts with no preview frames. I repositioned my antenna to stop this from happening.
If the program has ended and the preview frames are not available, you can do a reboot of the Tablo by briefly pressing the reset button on the back of the Tablo until it starts blinking. Do not reboot if you are recording other programs and do not hold the button down too long. If this does not trigger the creation of the creation of the program frames, you can still watch the programs using the timers. I hope that I explained this correctly. I am trying to summarize the help that I found from others.