Well, "nailing jelly to the wall" is a pointless, impossible task. It can't be done. Every time you hammer the nail in, the jelly would just fall to the floor again.
Presumably, writing a novel has proved or is proving impossible for him.
It's probably not the idiom I would have chosen as writing a novel is a rather long-term activity. I would associate "nailing jelly to the wall" with something more short-lived.
Trying to keep chocolate in my fridge for longer than five minutes is like nailing jelly to the wall.
- For Teachers