We just migrated our Magento 1 site to Magento 2. At some during the process, the default cron group stopped running. This was after data migration and possibly right after the first test orders were placed. It's hard to pinpoint the exact time. The only error we can see related to this is:
report.WARNING: Could not acquire lock for cron group: default, skipping run  
We have tried truncating the cron table, removing cron and reinstalling it, flushing the cache, updated the default group to run on a separate process, and running just the cron group. Every time we ran the command
php-7.1 bin/magento cron:run --group default, the above error logged.
Initially the index group was doing the same thing, but resolved itself.
We only noticed that certain tasks weren't running when the order and shipping grids weren't updating.
We are running Magento 2 Commerce v2.3.1.