Feeding PSB

Don't know what to suggest because I have never fed PSB when its flowering. I give it a good feed when I plant it out.
You don't get side shoots until the main flower head has developed and it has been picked. I have got 12 plants (early and late) in various stages of development, and some of them are like yours.
I can see your point about wood ash but it takes time to get to the roots. If the plant is sturdy and green (apart from the bottom leaves which naturally die off) I would leave it be. Someone may have other ideas.