Assume the following is given in a published paper:
There is an important result.
The important result might contain a gap, as possibly some people agree.
The authors never respond to questions on that and so maybe they do not want to make it public that there is a gap, which can not be filled.
It is not likely to give a direct counterexample to the important result, but only to point out possible gaps.
What to do then? The problem is obviously that some people might quote and use the result in new papers. How about the idea that such gaps could be marked in the published version of the paper as a motivation for future research? Of course a recent editor of the journal should choose a new referee to decide whether to mark such a paper. Also I think this should not have bad consequences for the authors as there might be reasons one is not aware of.