What am I missing?

I optimized my client's business GBP. He is a plumber. He's ranking in the top 3 on the local map pack. However he's barely getting leads. According to the Google Search Console in the last 28 days: Impressions: 640 Visitors: 26 So the average CTR is 4.1%. I used GMBRadar to track his local ranking. It even says he's in the top 1 in several areas. What am I missing? Why don't they click on my client's site?