I agree with Boiler. The water is too cold for whatever reason, and it's not warming up. Leaking tanks will do this, as will really hot homes with a water supply that is very cold. I've never seen or heard of an insulation kit for the tank, but wonder if the tank itself is extremely thin? Is it porcelean?
I'd take the lid off and watch the water level to see what it's doing. You should be able to hear the water run if it's dropping, unless you have the valve open just a bit and the water flow coming in is real slow. Most leaking tanks make allot of noise with the water coming on and off again.
If it's one of those American Standard tanks with the big O Ring type flapper, good luck. I've found them to be great for flushing, but terrible for holding water. I've thrown two of them away because it was impossible to get them to work right.
Eddie