Shaking Method for Garlic
I've had a few times where it just didn't work well.

I'm starting to suspect that the variety of garlic is also a factor -- I've had days where it works great, and others where it barely works, and I don't think it's an issue with technique.    Unfortunately, I don't have a place near me where I can just go and buy many different varieties of garlic to test with ... the best that I can do is disctinguish between hard neck and soft neck garlic ... and sometimes I get ones with purple streaks.

It might also be age of the garlic, rather than just variety.

You also need the right sized bowls for the amount of garlic that you're preparing -- I suspect that if it's too large, they don't beat against each other while shaking; too small and they're not getting momentum before they collide with each other.

update : I just did a quick test ... all garlic came from the same head of soft-neck garlic that I had (bought a few weeks ago), gently prying them from the head (did not smash the head).  I used two variety of bowls : 


smaller Corelle bowls, about 6"/15cm across, 2"/5cm deep
larger metal bowls, about 18"/45cm across, 6"/15cm deep


I used clean and dry bowls for each test.  Each round of shaking was about 5-10 seconds.  After the first test, I did a slow count to 10 for each shake, to try to make sure I was being consistent.  I removed the naked cloves and loose paper after a given shake cycle unless otherwise noted.  Results are as follows:


4 cloves, small bowls.  After the first shake, three were clean, one wasn't (and it was wide).  Two more rounds of shaking, and it didn't look like anything was happening, so I added back in the three naked cloves.  One more round, and it was naked.
4 cloves, large bowls.  As I suspected after #1 that shape of cloves mattered, it was 1 wide + 3 more roundish-ones.  After the first shake, the wider clove was naked, and I removed it and the paper.  After the second shake, all were clean.
1 clove, small bowls.  Took three rounds of shaking, but it did come clean without additional cloves added.
2 cloves (one wide, one round), small bowls.  After the first round of shaking, I discovered that the round 'clove' was actually the middle of the head, and was 2 small cloves (one of which was particularly tiny), the larger of which was now clean.  I left it in, and after the second shake, the wide one was naked.  4 more rounds of shaking, and I added back in the other 2 cloves.  Two more rounds (8 total) and I noticed that the paper on the small one was loosened enough that I could peel it free without much effort (but it wasn't naked on its own).
1 wide clove, small bowls.  After 2 rounds, the paper was loosened that I could've peeled it easily; one the third round it was naked from only shaking.
1 round clove, small bowls.  Naked after 2 rounds. 


Analysis : Oversized bowls don't seem to be an issue (undersized may still be).  The large, wide cloves seemed to be an issue at first, but peeled easier in other rounds than the mid-sized cloves; a small, tiny clove gave the most problems.  Although a single clove did require more time, and multiple cloves seemed to help, a single clove is not impossible using this method.

As they all came from the same head, I have no idea how much age or variety of garlic is a factor.  I'd say that I've removed the 'smashing the head' as a problem (as I did 6 rounds without it)