Why is some data missing on google finance for historical quotes

From the perspective of a data vendor, the answer is quite simple. This should not occur. We have multiple data sources for US stock data, and none of them showed any issue for NASDAQ:STWD on 20170731.

The only logical conclusion is that there's an issue with Google Finance. You should contact them for support to fix the omitted data.

NASDAQ:STWD had the following Open, High and Close for 31 Jul 2017: Open: 21.96 High: 22.09 Low: 21.88 Close: 22.04

Data from both Yahoo Finance and Google Finance have been unreliable for over a year. Suggest that you investigate other financial data aggregators; some very good free ones are Tiingo, Barchart, and Quandl. Each has a very easy API (application programming interface) and they only require you to register an email address. There are restrictions on the amount of data you can grab everyday, but those limits are very generous for a single investor.

I am not associated with Tiingo, Barchart, or Quandl. While initially using Yahoo and Google finance as data feeds, I have now switched entirely to the other three. Their data and API is much easier to deal with.