Well, Dave already kinda said what I was going to say, but I'll post it anyway.
People often confuse the ability to just draw what they see with nothing but memorization. While that is true in some cases, most of the time it's through study, not visual memorization.
When it comes to drawing, people seem to think that as long as they can picture it in their mind, but then they cant get that drawing down on paper, then they must have some kind of mental artistic block. Yet when applied to any other situation you can clearly see how absurd this is!
Imagine I told you to look at a car engine. Stare at the car engine, look at it nonstop for a week. If I then gave you an empty chasse and said "build me an engine" you'd say I was insane. You might be able to get some basic parts correct based on visual memorization, but without studying how the engine works there’s no way you could know how to put all of it together.
It’s exactly the same with drawing. People don’t just study muscle groups because they cant think of anything more exciting in their drab, boring lives. No they learn it because its necessary to build a drawing!
If I ask you to build an engine, you are going to get to a point where you ask “How does this valve fit into the engine?”
If I ask you to build a boat, you are going to get to a point where you ask “How does this pole fit into the deck?
If I ask you do draw a cow, you are going to get to a point where you ask “How does this leg fit into the torso?”
Now the reason why you might find that the better the artist the more they are able to just whip up a drawing straight from their mind is because a lot of their previous study has crossover like muscle groups and how skeletons are put together. They begin to get quicker at breaking an animal down into simple shapes and study the animal/person/object at a very quick speed, but this comes with years and years of study.
So to sum this all up, no there is nothing wrong with you because you cant draw an animal from memory that you just looked at, just as there is nothing wrong with me because I can't build a boat from memory that I just looked at.
Hope that helps.
"You’re right you DO suck, now do something about it"