I've had that happen. You may not have a reached a full trace in your mixing. Once I got a feel for what a good trace looks like, I haven't had it since. The recommendation on gel is good too; although, in soaps where your intentionally prevent gel by freezing, this usually isn't an issue. Just to be safe, be sure to wrap your mold completely in a couple of thick towels to produce a more even temperature during gel phase.