I would guess that if you can get them some times & not others that you are not always asking him clearly enough. At first you should exaggerate your aids until he realizes what you what, then you can back off & refine.
Sometimes they do get lazy with their changes & you can give them a spank behind your new outside leg. They will probably give a good leap forward if you need to do that, so you might want to grab a bit of mane so you don't punish them for doing what you want !
^THANKS! I will be adding that book MIE showed to my Xmas list!
I just got that book a couple of days ago-terrific! I also got one called "The Handbook of Riding Essentials" by Francois Lemaire de Ruffieu that I like. I'm sure it is waaay to elementary for most dressage riders but it breaks things down in a very understandable way and has excellent diagrams. It helped me really get shoulder in and haunches in and the tips in there for working on flying changes have done wonders for me and my hunter-trained horse. Definitely get the one MIE recommended though!
I have a great trainer, and we've worked on lead changes some, but not alot, and we havn't worked on shoulder-ins.
He can do lead changes and simple changes. I can get a lead change out of him sometimes, just not all the time.
I promise if you start working on some lateral exercises and bending, your flying changes are going to be a million times better. You need to get his weight moving to that outside shoulder (what is about to become the inside) so that he can step through with his hind to do the change. There is also a great video on you tube (and I have a final very soon so I'm sorry I can't look it up for the link here) by Jane Savoie on flying changes and using bending to get him to change with the hind.
Personally, when I talk about exaggeration I am talking about teaching them/getting them to understand what you want. Once they understand you can become more refined. Not many horses are going to be able to do a lead change that is show worthy on their first few tries, & neither are riders.