I'm having the same problem with some of mine
though admittedly I got 10+ months out of this particular set before the elastic started to go.
I'm trying to work out where I'm going wrong too & my washing routine is similar to yours, Lisa. Starting with a cold rinse, then followed by a 60c full cotton wash, then followed by an extented rinse cycle to get rid of any remaining detergent.
I usually use Ariel bio to wash them with, ~50g/ml scoop (about 1/3 of the recommended dosage).
The only thing that makes me wonder if it's something other than my washing routine is that I've got older nappies than the affected ones that I've been washing in exactly the same way that are fine, elastic wise...