When you are customizing the design and save a preset it will be specific to that design. If you edit your custom preset for the design you will need to overwrite the existing one with your updated one.
When I do this I'm able to use the custom preset for my design without issue. After I create the form on the page I can then edit the design directly, or choose the preset to edit from the edit form option.
In regards to the preset you created for the design can you see it as the selected design preset? When you edit the preset does it have all of your edits or the defaults? Did you ever edit the default and save it over your custom preset? Please post back with some more info on these so that we can try to determine what may be going on with what you are experiencing. Something else that may be helpful would be to go into the preset manager and export the design that you made a custom preset for. I should be able to import this and take a look at it.