There's huge potential in genetic genealogy

According to a study cited by ISOGG https://isogg.org/wiki/Cousin_statistics the average British person has an estimated 5 first cousins, 28 second cousins and 175 third cousins. There's a lot of variation, from no first cousins to more than 80.
Imagine you found a family tree where every couple had the same number of children who went on to have the same number of children — and it applied across the whole family tree. The table shows the number of first, second and third cousins anyone in the tree would have if that same number of children was two and three. Call them the Two Family and Three Family.

Relatives	1C	2C	3C
Two Family	8	32	128
Three Family	18	108	648

The average British person in the study cited has more cousins that the Two Family and many fewer than the Three Family.

In the highly influential study known as the Shared cM Project Blaine Bettinger sought information on the amount of shared DNA found by various genetic genealogists. The number of people reporting matches in that study for first, second and third cousins is shown in the table below. Also shown is the number adjusted proportionately to the number of first cousins (8) in the Two Family. The huge deficit, which is even greater for third cousins, implies a huge potential for further discoveries if the causes can be identified and addressed.

Relatives	1C	2C	3C
Shared cM Project	1512	1590	1791
Normalized to Two Family profile	8.0	8.4	9.5

What to do about the deficit.
(1) Many people deliberately test known cousins. Further conventional research on neglected collateral lines may identify more distant cousins to be tested.

(2) A 10% deficit can be expected at the third cousin level owing to the random nature of DNA inheritance. Exploring matches for others in your tree who inherited a different random draw of a third cousin's DNA broadens the search.

(3) People test with different companies. Transfer your results to other companies where possible, and to Gedmatch. Note that MyHeritage with over 2 million tests in its database will cease free transfers at the end of this month. Take advantage. The DNA Geek explains the procedure here.

(4) You find a match but are unable to get a response. Keep trying alternative approaches. Sometimes a round about way, like asking the spouse of a cousin to persuade them to test, can be effective.

(5) You can't get a DNA match with someone who hasn't tested. Suppose there are 10 million people tested in the USA. That represents a bit over 3% of the US population. That's 97% of unrealized potential and even greater potential in other countries. More people getting tested will help; less costly tests will help; easy access to test kits will help; testing companies and others better addressing scaremongering over privacy will help.