Thank you so much for your feedback. Going back to check the source, they did, in fact, have the 'no desu' form, but having not understood it previously, I didn't pick up on it.
I think the full sentence I heard was:
さいしょから何もできないんじゃないですか。- From the beginning, you haven't able to do anything, have you?
My explanation is that seeing as the 'no' makes the 'dekinai' work the same way as a noun, it treats the 何もできない as a clause in the sentence さいしょから...じゃないですか。
So this might roughly translate to 'From the beginning, is it not that you haven't been able to do anything?', as in 'is this statement not true' (obvious it is true). Again, this wouldn't make sense without the 'saishokara' so I probably should have included that in my original question.
Because you said that じゃない is the negative form of です, I don't know whether it is grammatically correct for them to go together or not, but they were definitely there in the source.
I would really appreciate any feedback on whether this makes any more sense.