the smaller head on my hammer coral looks really weak and is not improving no matter what I do. I have tried increasing calcium doses and water changes. Any help help to get it better would be much appreciated.
Lighting? Flow rate? In my experience, hammers generally do best with a low flow rate. This allows them to really 'open' up. That little bunch may just still be exactly that, little and not yet ready to blow up. I position my hammers in the middle of my aquarium and use 1 single current usa LED light.
My hammer has been by far my biggest surprise in regards to growth. I started with 1 small bunch 4 years ago and now have 4 MASSIVE sections. I add ZERO supplements to my tank. I'm not denying they may help, but in your case here, I doubt lack of dosing is the issue. The video in my signature has some good shots of my hammers opened up and full if you are interested.
To recap - low flow, middle of tank, medium light.