Couple of comments, it needs dead heading, there are seed pods forming. In case you are uncertain, they are the pointy ones

Mainly I'd say drought, it's a large plant in a small pot. Try standing the pot in a large bucket full of water if you can find one

What has probably happened is the middle / core of the root ball has dried out and water just cannot get in there, and even though you water it just runs through but not soaking in, submerging in water for a few hours should ensure the entire root ball is watered

Grandad is correct. The pot is too small for the plant (not enough compost so cannot retain moisture) and its suffering from being dry one minute to being swamped with water the next. Best growing in the ground if possible.

Nice one I’ll try that immediately 🤙🏻