My answer "你们这个周末能来吗?" should have been accepted as well - in fact, I'd argue that it's the more acceptable answer in terms of how a native Chinese speaker would actually say it.
This has been the most problematic issue that this course has; the Duolingo team simply considers elongated time positioned first.
Actually, your answer is correct - vice versa.
Yes, native Chinese would be fine with speaking both. Like English, the order of the sentence only depends on what the speaker wants to emphasize (in which the emphasized would be put first)
Still not accepted. Reported it.
Not only is also acceptable, but other Duolingo exercises taught me to place the time adverb before the subject. So c'mon! Which one do you want me to use?
Certainly it should be accepted. Nonetheless I don't think it is "more" acceptable and it just depends on the context.
Please change the accepted answers to include "你们这个周末能来吗?"
Is there any reason why 能不能来 doesn’t suit this sentence (whereas 能来吗 does)?
No there isn't.
Both forms are pretty interchangeable. Using 吗 may be a notch softer but what really matter are intonation and expression.