Why is my subdomain occasionally not accessible?

I own the following domain name: joynr.co

I've configured a subdomain: promoter.joynr.co

But sometimes, it's just not working, like a DNS error. Just like if it was still propagating. But it's been a week now :/

Where should I start looking ?

my apache confd files ? .htaccess ? or some configurations at AWS ? It's hosted a AWS, and domain name registered at godaddy.

I've followed this threat to configurer AWS/Go Daddy

Also, I've configured the confd so when accessing promoter.joynr.co you browse a different folder/website than when going to joynr.co (in /var/www ).

.htaccess is empty

Many Thanks

Any help would be much appreciated :)

Thanks