Besides the fact that they're losing their shape and bending over
significantly, they also seem to be blistering -- little pockets of air and
liquid just under the outer membrane.
letting the soil dry before watering them again.

The problem you describe is called oedema. It is similar to cracking in
tomatoes. It is caused by the leaf cells drying out & then rupturing when they
are filled with water. You need to water the plant more often & don't let it
dry out so much.

Iris,
Central NY, Zone 5a, Sunset Zone 40
"The trouble with people is not that they don't know but that they know so much
that ain't so."
Josh Billings (Henry Wheeler Shaw), 1818-1885