The assumption is that the Pope would not order a Bishop to violate the faith but as Mike said, if something isn't clear, then a Bishop has the right, and indeed duty, to ask for clarification.
Such a request for clarification should be done in a respectful way and best if not done publicly to avoid scandal.
The presumption is that, if not infallible, Papal Encyclicals, Letters, Exhortations, etc. are authoritative.
In the case that Mike mentioned, Amoris Laetitia, On Love in the Family, the Encyclical was at points ambiguous and some offered a highly questionable interpretation, however some might argue that the bishops who questioned it, went about it in way that couldn't have been handled better.
As a result, they got silence or a non-response.