Problem reindexing through Magento CLI

Hi, guys,

I am a beginner in using Magento. Our shop runs on Magento CE 2.3.4 and I have problems reindexing through CLI.

All are reindexed fine, except one: Smart Category Product Rule.

I tried to reindex it individually, by running indexer:reindex smartcategory_product, but nothing happens. No error, nothing. I tried it several times and the same result.

In admin I receive the same warning that the indexer needs to be reindexed.

What could be the cause of this? 

Many thanks,

Robert.