Any answer here can only be speculation, but as you suggest maybe something went wrong when loading data (perhaps indexes weren't added properly and any query takes a very long time?).
What I'd do first is to see if the data that isn't rendering is actually present in your database using "psql". Pick a part of one of the borders that isn't rendering, query it on OSM (the "?" button) to find an example way id and see if it's in your data. For example, 306374624 is currently part of the Ireland/UK border, which appears to be missing from your data:
gis=> select id from planet_osm_ways where id=306374624;
id
-----------
306374624
If it is present, but not rendered, then try and understand under what circumstances things get rendered and not. If it is present but that query isn't instantaneous maybe some indices are missing? If it's not present maybe a data load failed, or a query deleted some data that it shouldn't have?