From what I can see, it seems the problem is that the water is running directly under the running lava. Try putting the water and lava on the same level and having the water drop one block a block away from the lava, like this:
If the water runs directly under the lava that's flowing above it, you only get stone. The water and lava have to be on the same level, but if you don't put a drop the water will run into the lava and make obsidian.
Hope this helps! If you have any questions, please ask.