Why did google stop indexing only my Blog pages?

Hey guys, a local plumber here has a site which was ranking very well for the past 5 years until last year when his Stats started slowly dropping each month. We were baffled by this as we were consistently still doing one monthly blog and monthly backlinking without missing not even month.

I only recently noticed that only his blog pages had issues all this time not being indexed so my fault in not seeing this sooner. GSC shows 59 blog pages not indexed so I reached out to a WP Coder who...

Why did google stop indexing only my Blog pages? WordPress ...