Yeast head

[Dr. B]

I have been reading Graham Wheeler's book and he talks about the protective yeast head forming.
I have brewed loads of beer but never had this.
The last beer I made I used Safale S-04, whereas I often use general purpose, or (if a kit) the yeast that comes with that.
I normally get a frothy bubble head which disappears after a couple of days.

Why am I not getting this?
On the other hand, my beer is turning out okay...

 

[RobWalker]

That's what it is - the froth on top protects it during fermentation because it doesn't let gas in.

 

[Dr. B]

Yes, but Wheeler says it should not be like a self supporting foam, but more like whipped cream texture.
I get the former.