#2 Personal Issues
Alexis recently noted he is splitting from his long-term partner Mayte Rodriguez. Players are human, and they have issues in their lives that can affect their performances like any other.
Moreover, maybe he doesn't like the north-west of England? Manchester is a fine city - but it doesn't compare to London. To be fair, very cities on Earth do.
But there could be a number of reasons why he hasn't performed well yet, which transcend the footballing scope.
Chile also didn't qualify for the 2018 World Cup, which was a shock given their recent successes in the Copa America. Alexis for the large part was a key factor in these wins, scoring the penalty that won them the competition in 2015:
As part of a golden generation for the country, maybe not qualifying for the World Cup was a blow that was hard to recover from.
Professional sportspeople should learn to handle defeat. But then they are still human, and defeats can still affect them.