Well Wireline Logs

I was reading a bit about well wireline logs and it sounds like the logs are collected by dropping sensors into a borehole and then collecting the measurements as the sensor goes deeper into the earth.

Does anyone know why there would be missing values associated with the wireline logs? Were the logs collected over a large number of years, during which some tools where unavailable?

Or, is it an issue of cost? Different instrumentation costs differing amounts and engineers make judgments about what testing is most important with respect to the cost of such testing?

Or, does the instrumentation often break and so logs are typically incomplete and it takes several passes to get a largely complete record? Or, records are typically incomplete but human judgement is enough to deduce the missing values / the likely lithology at that depth?

In short all of the above:
Typically in order to save cost one runs the full logging suite only over the key reservoir interval. Hence there are often more incomplete logging suits in the overburden. In addition you often have tool failures which leads to data gaps
Where the data is missing completely lithology is interpreted by looking at the rock that is coming out of the borehole. This data has been heavily relied upon in the preparation of the lithofacies dataset that we shared.