If a plugin's graph_category is written in all lower-case, they won't be shown on the HTML page if one or more plugins in the same category has its graph_category written with (at least) the first letter capitalized.
In my case, I had a few intitial plugins with graph_category postgresql. Then I introduced a few more plugins, and for no reason I defined them as graph_category Postgresql - and then the first plugins disappeared.
The logs indicate that all plugins are being executed and that the data are properly collected, just not included by munin-html.
The plugins reappeared in the HTML page after changing the initial plugins' graph_category from "postgresql" to "Postgresql".