Lava and water won't create cobblestone if the lava has already flowed completely.

Steps to reproduce:

1 - Place a lava source block;

2 - Let it flow completely;

3 - Place a water source block anywhere near the lava flow;

4 - Cobblestone won't be generated as it should.

In the pictures, a redstone block represents the position of a lava or water source block.