I presume it's Solid Water that you're using? Scenic Water should harden in just a couple of minutes, as soon as it returns to room temperature.
Anyway, if it's the mixed resin these can be a bit temperamental, assuming you got the proportions right you might just need to be a little more patient - maybe an additional 10 or 12 hours before you really start to worry. I'm always getting much slower setting with two-part products (as much as double) so I don't trust the times given at all any more!
If it is going to set and it's just being slow putting the base somewhere warm will help.
Einion