As far as I know, you need an additional tag which sets the mobs health. It has increased max health, but still has it's normal health amount. I think Health:80f as an additional tag should solve this.
Edit: This is the command we used for a shulker with custom health.
I have this command, i want it to have 80 hearts, but it does not work. if you know how to fix it, tell me how! (Btw, this is 1.9)
/summon Zombie 121 5 -107 {CustomName:"Lurker",Attributes:[{Name:generic.movementSpeed,Base:0.3},{Name:generic.maxHealth,Base:80},{Name:generic.knockbackResistance,Base:0.2}]}
As far as I know, you need an additional tag which sets the mobs health. It has increased max health, but still has it's normal health amount. I think Health:80f as an additional tag should solve this.
Edit: This is the command we used for a shulker with custom health.
/summon Shulker -65 159 -88 {NoAI:1b,Glowing:1b,Attributes:[0:{Base:100.0d,Name:"generic.maxHealth"}],Health:100f}
The d and the f may be important, I'm not sure it will work without them.
see what this does
/summon Zombie ~ ~1 ~ {Health:1000.0f,Attributes:[{Name:generic.maxHealth,Base:1000.0}]}