The first question I guess is what have you actually deployed - the version of OSM Carto at OSM, or the “next” version as being developed on github?
OSM Carto is / has changed from using the pgsql backend of osm2pgsql to the flex backend, with a different database structure. I’ve no idea whether the wiki page you followed knows about that. More reliable resources may be the github documentation at OSM Carto itself, or the switch2osm.org documentation (which uses a pgsql backend for now).
Another question is are you consuming minutely or other updates?
That said, if tiles used to render and they don’t now then you ought to be able to look at what is failing. The switch2osm.org guides explain how to enable sensible levels of debug and you should be able to see that in syslog or via journalctl or similar.
An “intermittent” problem might be “out of memory”, so that is something else to look out for.