Clarence
Landlord.
Not sure @dmtaylor says he has a grain father, but agreed if grain father user should be able to do an overnight temp mash at a set temperature.
The OP has a one, I think. I've never used an all-in-one, but isn't the temperature control governed by a built in thermostat? If the OP is relying on that for his mash temperature then that might be the source of the problem. Do these Grainfather's circulate the mash from time to time? If not, the heating will be quite localised, won't it.I have been brewing using an original Grandfather (30L).
Just some thoughts. I'm a cauldron and plastic bucket man.