A bit of everything, 5 have been stopped due to cracks: https://en.m.wikipedia.org/wiki/Stress_corrosion_cracking

A few are having their once in a decade maintenance check

Some are waiting for maintenance which were delayed due to covid

We didn't train enough maintenance workers because everyone was talking about shifting out of nuclear in favor of renewable (we're using American contractors now afaik)

So mostly bad decisions and poor management, it's a political issue more than anything else, it worked fine since the 70s to more or less a decade ago

Nothing is "unreliable recently", the mess is decade old, simply when EDF was private, after have acquired public made energy systems, they reduce investments to the minimum creating the present mess. Once the damage was so big they can't mask it again they left, leaving the mess to the public.

The public, who is abetter, have managed to push some excuses to avoid finger point any private friend and so reveal the mess all at once.

In the summer they had lots of cooling issues due to low water, if I remember the news correctly.

The article speaks of "ongoing or delayed maintenance, or corrosion problems" as the next reason for the problems.

Short answer: Lack of investment caused by incompetent politicians, supported by public opinion. Lot of French are anti-nuclear.