What “gap” could have caused the Starliner instructions to be delayed?

Question

The Starliner launch didn't go as planned today

After being released by the rocket, Starliner was supposed to use its Orbital Maneuvering and Attitude Control engines to provide the thrust needed to reach a stable orbit and begin the process of catching up to the International Space Station. But that did not happen.

What intrigued me was this part

When ground-based controllers realized the problem, they immediately sent a command to begin the orbital insertion burn, but due to a communications problem—which could have been a gap in coverage of NASA's Tracking and Data Relay Satellite System or some spacecraft error—those commands were not received right away by Starliner. So it continued to expend fuel to maintain a precise attitude.

What do they mean by a gap in coverage? Is there some part of orbit that cannot be reached by a satellite or a ground station?

Answer 1

This was briefly discussed in the Dec 21 press briefing.

It wasn’t a gap in the sense of “no radio waves here”. Rather, since the craft was confused about what time it was and hence what attitude it should be maintaining, it was also using antennae that weren’t optimal for acquiring the signal. The signal acquisition was slowed down due the resulting low signal strength.

More detail wasn’t presented, and my knowledge of TDRS is quite old, but it seems likely that the craft might have had to e.g. drop back to other antennae, perhaps low gain ones, when it didn’t have enough signal to get through the complicated TDRS lock sequence (see section 2.4 here).