One of the option not covered in the other answers is that the problem stated requires skills/knowledge beyond yours to understand or that the answer is exactly that you tell that it doesn't make sense. As an Interviewer I regularly ask questions which seem trivial to people not trained in a specific field. A typical thing would be controller design where there is a simple layer (Test if that works in a simulation), a medium layer (Analytical criteria for stability) and a abstract layer (Coupled nonlinear systems).
In such cases it very often happens that the candidate believes that the questions is answered completely and thoroughly, yet it just tells me that he/she didn't go to the bottom of it. And it actually could be that "ok, thats the function you asked me to implement, but don't put that in a production server because the database design is not normalized and you will probably get an inconsistent database" is the answer they are looking for (or in my case, yep, the controller is the design you asked me to do but please never use this....).