A quick pre-answer is to suggest that you look into the idea of probabilities. While infinite possibilities exist, we are limited to experiencing infinite probabilities :)

If someone is already leaning towards the idea of experiencing themselves as a world class athlete and their actions and behavior are not in conflict with their desires, beliefs and expectations, then there's no reason why they won't achieve their desire. After all, we are all already everything, but we must tune-in to any particular reflection in order for it to manifest in physicality and thus, BE realized as our reality.

Conversely, if someone says that they desire to be a top athlete while at the same time their action consists of playing the role of a couch potato who curls up on the sofa every night, whilst watching sports and demolishing a six pack of beer; it's unlikely or improbable that they'll achieve their desire. In fact, at some level they will know this and so they won't really believe in their own desire anyway. At any rate, their actions and behavior are in conflict with their desires, beliefs and expectations, so they won't get what they want.

I'll just add that when considering ideas and concepts such as the LOA and manifesting, it's important to understand that there are several elements to address and it can take time to realize this and to bring all of the elements into balance and harmony or alignment. It's helpful not to get caught up in one area and dismiss the whole concept, just because it may not be working for you at this time. There may be other elements that you haven't quite understood or are not quite ready to allow yourself to remember at this timing.

Do [you] 'masters' of the LOA get everything they [you] desire?

Well, I wouldn’t label myself as a master just yet (even though I know we’re all masters), but to answer your question, yes I’m getting everything I desire and in addition, things I didn’t even know I desired, but am more than happy to receive.

One of the keys to manifesting is to realize that a) you desire something believing that it’ll make you happy and b) you must be happy before your desire manifests, otherwise it cannot manifest.

My preferred method:

My first goal was to bring myself to a state of happiness, which equals feeling good and total well-being. And then, a while ago I refrained from insisting on a time-frame in which my desire must show up. My attitude is now one of believing and expecting that only good will come to me, while relaxing and enjoying my life. I guess I’m allowing Universal synchronicity to unfold in perfect timing...

Caveat: Once any particular desire has manifested in your life, it’s important not to become attached to it. But note that this is not the same as the Buddhist idea of cultivating non-attachment.

Use whatever method and whatever tools work for you...

Be Happy Now and Feel Good Always then Only Good Can Come To You - SURE :)