Any of you remember the old days? Hand times. Long tight, jammed chutes. Results days and even weeks after the race. No start or net times. No splits. No way to catch cheaters. With that said, just remember how far we have come.
This is a topic not a lot chip timers or chip equipment manufactures will say a whole lot about. Everybody wants to get and keep business. A lot of people will lie to themseleves and to others, to prove that what they have is the best. No chip timing system is perfect and nobody gets 100% reads, no matter what anyone says. With our 134 kHz system, I figure about 99.85% with one system, and 99.98% reads with backup. Almost always, those missed are wearing the chip incorrectly in some way. Wearing it next to a key on their shoe, holding it in their hand, tucking it in their bra (I have seen this way too many times!), and yes, sometimes they just don't work. Metal, underground fiber optic or electrical lines in the road way, or large sources of EMI can cause problems. These reasons will account for intermittent reads. Also, it will absolutely amaze you how many people will go around a timing system. Unless it is barricaded, people will step over around and thorough cones or people.
Even the orientation of the direction the chip is facing can affect the probability of it reading.
Sometimes, workers are just stupid and forget to turn the system on. It is easy for them to just blame it on the equipment or the chip.
The standard is to use primary and backup systems at the start and the finish. Sometimes timers won't use a backup system on split points. It is just a way to save on equipment. That is why you generally see more skips on the splits.
Most chip timers have moved away from any back up timing method. They usually say "no chip, no time." Those who do use back up methods usually only use them at the finish line. They can use one of many methods. Time Machine, Time Tech or Chronomix getting times and select bib numbers. Photo or video. Even handwritten sheets. It all depends on the size of the race and how much effort someone wants to put into it. Most timers are lazy anymore, don't use any back up system, and completely rely on this chip system.
If you see someone with skipped times again and again, another thought is to ask whether they are consistently wearing the chip incorrectly. Or maybe, she has a personal chip that is malfunctioning. I highly doubt that from what I've read of Ms. Noble, though.
Speed of the runner and density are two factors that can lower the chance of a read, as well. The hardest time to record is the start. It has the highest density. Next hardest would be the finish. For the most part it has the highest speed (not necessarily in marathons). Each of the different timing systems has its own advantages and disadvantages with regards to its ability to handle speed and density.
Hopefully all this helps to answer the question.
Probably the best authority on this is David Katz. Maybe he can sign in on this and lend his thoughts.