From my fairly limited experience, if you're using dichloromethane as a blowing agent, this through time can degrade some of the mercury catalysts used for PU, so you may need to add some more catalyst to your polyol formulation. If you're not using any dichloromethane and your system has slown and you can't find any temperature changes as Michael pointed out I'd slowly increase the catalyst concentration. Also increasing the temperature of the mixing head and of your mould tools should also help speed up the reaction.
...no water is evolved. It can be an outside contaminant, though, especially for an opened drum of a hygroscopic polyol...
True, but its not due to an equilibrium reaction condition. The foaming mentioned in the presence of water is a consequence of water reacting with the isocyanate to evolve CO2 (via a carbonic acid intermediate...) and an amine terminus is left behind at the former isocyanate site. The primary amine (vastly preferred over the polyol) can go on to consume yet another isocyanate and form a substituted urea, which chews up yet _another_ isocyanate to form a biuret.
Fortunately, primary alcohols are preferred, even to water, but of course they'll compete to some extent if isocyanate is present, and if the polyol hydroxyls are somehow sterically hindered (high MW, low OH functionality?) the water can become the preferred reactant. Secondary alcohols on the polyol will definitely react after the water is consumed.
So yeah, in short; Water Bad. Measure water content on the suspect polyol side of your two pack by Karl Fisher. If you've got more than 0.25%, you've found a contributing problem.
Another thing which can retard urethane cure is old isocyanate, particularly if it has seen elevated temperatures. Self reaction to form uretidones and cyanurates might slow down its cure rate by the time you get to application. Check the lot number and history on the iso side, and see if it sat in a warehouse for a summer...
I could have sworn it was a polycondensation reaction, but I stand corrected. This is what happens when an engineer attempts chemisty. Especially when his reference books are at work and not home. :-).
I dont actually think changing the mix will slow down the curing of the reaction if anything it will change the material density after curing! You maybe better contacting your supplier and telling them you want a slow curing mix, you need to know how long a cure you want before doing this.
Beng Polymer Engineering Msc Manufacture & Design with Polymers