Posted: 28/06/2012 at 09:25
As you suggest, I don't think over-watering will be your problem, in fact the conditions seem ideal. Here in the UK sweet peas don't have many insect enemies, but I'm not really familiar of what you have in N Spain.
In view of the fact that your other 2 wigwams are flourishing, I'm inclined to think that there maybe something lacking in the soil.
Try a feeding with a foliar feed of liquid seaweed (hope this is available where you are) this is best used as a tonic and not a regular feed and should be discontinued after they perk up.
Other outside and unlikely causes could be: Pea mosaic virus, powdery mildew, root rot, or herbicide spray drift.