This idiom, itself, will be lost in translation.
I agree with the suggestions already given although they do not completely capture the idea that some nuance is missing in a translated text.
Often, there is no direct equivalence between languages and this limitation becomes more apparent the further you get from plain declarative sentences. Thus, my suggestions will likewise fall short of the target:
Hindi lubos na pagsasalin.
Kinulang sa pagsasalin.
Hindi hustong pagsasalin.
Kinapos sa pagsasalin.
Hindi wastong pagsasalin.
Hindi bagay na pagsasalin.
Naligaw sa pagsasalin.
My preference would be for Naligaw sa pagsasalin as this more closely adheres to an idiomatic structure in that the true meaning is not directly deductible from the individual words.