Shokin didn't catch corruption. It was his deputies who were corrupt, and caught by the national police.
Here's an official CONGRESSIONAL report from the time.
chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://www.congress.gov/116/meeting/house/110331/documents/HMKP-116-JU00-20191211-SD046.pdf
MOSCOW — Bowing to pressure from international donors, the Ukrainian Parliament voted on Tuesday to remove a prosecutor general who had clung to power for months despite visible signs of corruption.
The United States and other Western nations had for months called for the ousting of Mr. Shokin, who was widely criticized for turning a blind eye to corrupt practices and for defending the interests of a venal and entrenched elite. He was one of several political figures in Kiev whom reformers and Western diplomats saw as a worrying indicator of a return to past corrupt practices, two years after a revolution that was supposed to put a stop to self-dealing by those in power.
With the prosecutor’s office in turmoil throughout Ukraine on Tuesday, one of Mr. Sakvarelidze’s appointees in the Odessa regional office was arrested by military prosecutors, assumed to be loyal to Mr. Shokin. Foreign donors had complained about rot in the prosecutor’s office, not least because much of the money suspected of being stolen was theirs. In one high-profile example, known in Ukraine as the case of the “diamond prosecutors,” troves of diamonds, cash and other valuables were found in the homes of two of Mr. Shokin’s subordinates, suggesting that they had been taking bribes. But the case became bogged down, with no reasons given. When a department in Mr. Shokin’s office tried to bring it to trial, the prosecutors were fired or resigned. The perpetrators seemed destined to get off with claims that the stones were not worth very much.
