I realize that Charlemagne was a pretty brutal guy at times, but then there is Olaf II of Norway. Given his contribution to the church and the number of converts he produced, he seems like a shoe in for the medieval church.
Someone with greater medieval church expertise can probably come in with a more detailed response but I can give a brief one.
Charlemagne was canonized in the Middle Ages but it was done by the Antipope Paschal III. The canonization, along with all other acts by Paschal III, was nullified at the Third Lateran Council in 1179. I don't know why the idea never came up again after that but no doubt the lingering connection to an anti-pope did not help him much.
The reason for his canonization was that Paschal III was in a disputed papacy. In 1159 Alexander III was elected pope but some of the cardinals disagreed with his election and elected a different pope (Victor IV) instead. Victor IV was followed by Paschal and he in turn by Calixtus III. This line of anti-popes was supported by Emperor Frederick I 'Barbarossa' as part of his ongoing dispute with both the Pope and the Northern Italian City states. Charlemagne was canonized in a sign of support for Frederick since Charlemagne was seen as a predecessor to Frederick, both having been Emperors in Europe crowned by Popes. Eventually Frederick had to give up due to mounting costs and failures in his Italian campaigns. Frederick recognized Alexander III as pope in 1177.
Most of this I got from lectures by Prof. Ian Robinson so I'm pretty unqualified to recommend good books on this particular topic. His book The Papacy, 1073-1198: Continuity and Innovation is probably a good place to start though!