I am not an expert on veg growing but grow these as an ornamental - they are big thirsty plants, are you watering enough?

I have them in my flower beds for their magnificence. We have had plenty of rain and they have never been a problem during past years when it has been dry.

I've zoomed in on your image but can't quite make out the centre of the plant but can clearly see the leaves flopping over onto the ground. If this were my plant, I'd still be inclined to think it needed water.
Have you checked for slug and snails causing damage in at the crown - I've been picking the snails from mine on a daily basis this last week or so. Earwigs can also be an issue.
I'm at a loss to offer any other advice because other than the problems I've listed I've found no other bother with mine. They even coped with flooding back in 2012.