I just want you to know that I'm a beginner in management (having hired two juniors for my startup), and I'll use this advice (The junior is not productive, but him being reasonably motivated is the reason why I'll keep him).
I'm somewhat playing devil's advocate here, but how is your startup's documentation?
Sometimes juniors have a hell of a lot of trouble admitting that they don't understand something. Consequently, they won't be productive because they not only don't understand what they're doing, but they don't want to demonstrate their ignorance.
If I were in your shoes, I would do a couple of things:
1.) Make improving documentation a big part of their improvement plan.
2.) Make it very clear that you value clarity above all else and actively encourage them to tell you they don't understand something.
Conversely, ramp-up time should not be infinite. A junior employee should not be limping along after a year at the company. Either that person just isn't a fit, or there is something terribly wrong at your company.
As a step before firing for a junior employee that shows any sign of promise, a 3-6 month performance improvement plan with regular check-ins and measurable progress points could make a world of difference. I currently have multiple juniors on PIP's and they are taking them very seriously and improving less than acceptable behaviors.
