DennisW4, while the reasons why some properties are, or are not, covered by the CCR's in your area differs from our situation, there isn't really much you can do about it.
1. Is it well known that some homes are part while others are not? ie. at time of purchase.
2. You obviously bought into the neighbourhood when the situation existed yet it didn't hinder your decision to buy. If there was something outrageous being done by the non-members I would think you would have noticed it visually.
3. As mentioned, it remains up for debate whether living in a HOA 'protects' property values.
4. These folks have apparently been there longer than you and your HOA so just leave them alone, unless you have something to offer them to entice them into joining your HOA.
5. Making waves and upsetting non-members could just backfire. You can't park your boat or rv but there is nothing stopping me from offering my driveway to you to do so should you need.
The last comment shows the potential for lack of uniformity in appearance which could be a problem for some. ie. Some uneducated HOA member could use the common line, my neighbour is doing xyz and not getting a violation notice, but I did, without understanding the distinction that HE chose to buy a deed restricted property whereas his neighbour hasn't.