"You try again."
Translation:Jij probeert het opnieuw.
I have the same question. To try again and to try something again are different things in English, if only slightly. Actually, there are example sentences, where the direct object is omitted, and they seem fine (though I'm not sure if they are confirmed in any way).
To refer to the sentences you link to
- Probeer opnieuw - Borderline case, since it is an order it is clear what you have to try again. Though probeer het opnieuw would be clearer.
- Probeer met alle middelen. - This is not a complete sentence in Dutch and will lead to the question, what do you have to try by all means? It's just a literal translation from English.
- Je moest proberen. - Again a literal translation from English, this is not a complete sentence in Dutch and will lead to the question, what did you have to try?
So in summary, if you leave het out, you create the question: Try what (again)?