There are 2 ways to go ground-to-air.
1. Prior to commissioning, change your contract with the OSO. HQMC gives each OSS air slot quotas, which he usually fills with incoming candidates. For him to give you one of those slots, he'd have to take that chance away from a new guy. Granted, each OSO operates a bit differently, and he might think that you (as a "salty" candidate) deserve that slot over someone else, but that's something that only the OSO can decide.
2. During TBS, air slots often open up from guys DOR/NPQ/attriting from flight training. Now, in a company of 200-300 2ndLt's, 1-5 (average) air slots just opened up. As you can imagine, they are quite competitive, and there are many threads on the forum detailing how to compete for them.
3. A select few Marines "know" somebody in a high place that may recommend/approve a "lateral move" to a new MOS, namely flight student. Unless you are related to a Colonel or above, this is probably not an option for you.
As far as knowing guys who didn't get that air slot they wanted, yes, I personally know several who tried each route I described above and failed. There are countless others on this forum that are in the process and will likely not succeed. Almost all of them had/have one thing in common: their OSO told them that they'd be able to pick up that air slot with no problem.
The bottom line is, you are encouraged to try, but if you don't get to be a pilot, are you still willing to <sniff> be a Marine Officer</sniff>?