Grafana shows N/A while connecting to few Zabbix despite correct setup

We’re running Grafana v6.5.3 (commit: 05025c5).
We monitor data from few different Zabbix. We’ve created many singlestat dashboards dedicated to only one project with the essential data for us. And from this we wanted to have one big singlestat dashboard with the status of calculated metrics in Zabbix.
However, data doesn’t load correctly - it seems as if Grafana was blocked by the amount of different connections (our wild guess) and simply gives N/A on a few panels (please check the screenshot, I marked missing results


And this is not metric-related: if the order of the metrics is changed then different ones are not displayed. What’s more, N/A appears automatically, as if Grafana wasn’t waiting to collect the data (loading ‘bar’ is not turning)
In logs we haven’t found anything that could tell us what’s wrong. Machine on which Grafana stands has enough resources (memory, CPU are barely used even while loading the biggest dashboards)
For now as a workaround we created a playlist with two smaller dashboards where all data is being gathered with no issues.
Have you encountered such issue before? Is there anything else we could check on our side? Maybe you could check how Grafana behaves when collecting data from few different Zabbix at once?