I don't believe there is a solution for this, if no one is bidding for placement on the site then google won't show any ads. Is the site new ?
I always start out by adding minimal ads to pages, having a single ad slot can create higher cpc as bidders only have a single ad space to bid for.
Also have you adjusted the AD balance in your AdSense settings?
I am using only 2 ad units now, and they are showing ads. Yes, you are right, fewer ads make more money.
One of my websites gets 1k visitors a day, however the revenue is very low (around $4) I have tried auto ads and ad balance they didn't do well.
what do you suggest?