What techniques or tricks make soft, flaky pastries instead of leathery ones?
There are two ways that I make puff pastry. The first is Rough Puff Pastry which is a quicker way to make puff pastry and you get about 75% of the rise you get with normal puff pastry. 

Here is a link for a Rough Puff Pastry that I use. I'd say that the most important tips to take from that recipe is that the butter must be cold when you start and make sure you're using really cold water to mix in. Using a knife instead of your hands is also a good idea to stop your hands melting the butter too much - you want to see the pieces of butter in your pastry. Making sure you chill the pastry after folding for an hour or as long as you can.

Secondly, to make 'real' puff pastry, you need to have to plan in advance as it is going to require a lot of chilling. Firstly, make your dough separately, roll in a ball and chill overnight. Then roll out your butter and make sure you then chill the rolled out butter again. You then continue with the folding and chilling as before. Again, the most important thing here is to chill for at least an hour in between the folds. This ensures that the layers of butter and pastry build up, and that is what gives you the best rise and the flakes you are after. Here is a good recipe that I use with lots of other tips and tricks in.

From looking at your pictures, in my opinion, the 'leatheriness' you are seeing looks like the pastry is a little under baked. The 'tough crust' looks to me like there aren't many layers of butter towards to the top of your pastry which is why it hasn't puffed out when it's been baked. Definitely try following one of the above recipes - if only to get the technique right - and go from there. I definitely find the trick is lots of chilling and keep everything cold! I sometimes even chill the flour!!