Hey Coen,
There is nothing much to it, so the only thing what can be going on is that you have a mess in your folder structure. The actually only motto is : make it logical so that your own eyes can not be confused, then XXHE understands it too. For example, putting two albums in the same folder is the opposite of this. But you knew that.
Otherwise this post may get you going :
http://www.phasure.com/index.php?topic=2012.msg20902#msg20902In addition (but this is more extended), you can try to read this topic too :
Problems with Gallery creation.
I hope this gets you going a bit.
Peter
PS: Take care that no "coverart" or anything is accedentally there at a higher level. That too will mess up and is a typicle reason for only a first album showing (that wrong) Coverart.
PPS: Nothing is "one by one" because you can select ranges (or everything) in the Library Area.