EXACT same situation and I'm almost positive I've figured it out, just recently actually. Pretty sure their algorithm if you don't pay is based on the overall score of the business.

First, it almost certainly has nothing to do with the reviewers themselves apart of the standard IP check. I've seen positive reviews from someone with 20+ reviews and many friends removed while positive reviews from someone with one single review (this review I'm referring to) and zero friends have their review stick.

Recently I started working with a business that has two locations and two Yelp pages, one had 7 reviews at 3 stars and the other with a few more at 3.5 stars when I started. Then the first got a positive review, 8 now total, and their overall score went up to 3.5 stars. Ever since that both locations have hovered at 3.5 stars. Luckily we haven't received a negative review at either location in a while on Yelp. But the point is that I never really noticed this before because the businesses I've worked with in the past had many more reviews so each individual review was weighted much less. Every five-star positive review we receive on either page bumps the score to 3.8 - 4 stars then we either lose that review or an older positive review and go back down to 3.4 - 3.5 stars.

My theory is that you can't get above 3.8 - 4.0 stars without paying and if it's possible I'd love to know how it's done. We've had real customers tell us that their review was removed from Yelp, happens often.