This is usually caused by wetness and some look fine. if there had been a deficiency I would expect there to be other symptos.

Mine go like this and I just take off the brown flowers.

Mine do this sometimes I thought it was just spent flowers and pick them off. Dont over water them though.

I put this down to lots of rain. We're never satisfied, are we! :-)

Yes some mine go like this after all the rain ,but I keep dead heading to tidy them up.

Now all are indoors for protection to the weather.