The mane is determined by genetics. Lion heads come in single and double maned. Single manes are just around the neck while double maned is around the neck and the skirt (around the flanks). Single manes can be lost with the first moult, but some do stay. Double manes stick around.
Some lion heads just don't have the same poofy mane and others. This is usually caused by breedings that are not done to the breed standard. Many breeders are breeding to the standard, but some aren't and seem to just want cute pets.
It would seem that Cleopatra just doesn't have the full mane. There is nothing wrong with this, just how she is.
I doubt the mane will get thicker with age. At 1 year old, she is pretty much what you are going to get.