Any photographic trip involves an element of chance, and some subjects are obviously a higher risk than others.  This is not always a problem in that sometimes it's a matter of going to the right habitat and seeing what you can find and then photographing that subject.  Africa is a good example of this, there are some great species which attract tourists, wildlife watchers, and photographers from all over the world, but what you actually see always involves an element of luck.
I was more than happy with this, and I am happy to photograph whatever species that our guide found, but... one species I have always wanted to see is Cheetah, and during my two visits to Africa in 2012, I was really pleased that we saw Cheetah on several occasions.