Certified GKH Gardening Expert
I'm seeing signs of overwatering stress. The soil looks very heavy with a high clay content. This will, often, lead to problems with agave. The soil is best amended with lots of sand, and some compost, to give it a much lighter texture. Unfortunately, clay soils can hold too much moisture under the surface.
Make sure that these do not get watered unless there is a very severe drought. If it rains often in your area, then this may be a battle for the remainder of the plants' lives.
It may be best to use biofungicides to counter the moist soil conditions.
Here are some articles that will help: