Recently I've been running the SSH command line php bin/magento indexer:reindex
But twice in 3 days it starts to hang after Catalog Product Rule index has been successfully rebuilt..
My server ticket support told me "There are a few reindex processes locked by another reindex process"
What does that mean and why does it happen?
The ones I can see were locked on SSH were: 1. Customer Grid Index 2. Design config grid index