Something if "off", either:
C:N ratio- brown-green mix less than ideal
Moisture- it's too wet or too dry
Air

Or, could it have heated already and then cooled? Mine stays hot for a week or less if I don't touch it.

You just have to play with it to get it right. Maybe try adding nitrogen (green) and see if that works, give it several days. If not, maybe try more carbon (brown). Do you think it's too wet? Flip and let it dry out some. If it seems dry, try more water.

Even when you get the right knack, sometimes it might not get as hot as other times. It's really a matter of experience, and doing it is the only real way to learn. It's one of those "practice makes perfect" things.

But there's nothing wrong with cold composting. It will still break down into good stuff, it just takes longer.

Karen

Don't stress about this.

Sometimes it just takes time for things to get cooking.

The presence of flies tells me the material may be too wet. Too much moisture excludes the air the bacteria need to function and the compost will then go into anaerobic (in the absence of air) digestion which does not heat up.

I wouldn't expect it to be hot a month later unless you are adding significant quantities to it regularly. Are you doing this? Flipping the entire pile once/week or so? Is the size approximately 3x3"x3'? or bigger?

Karen

I'm not sure flies always = too wet.

I just made a ton of marmalade, and tossed the white pith on the pile.

The pith isn't particularly damp, but I got Instant Flies.

In a properly built compost pile the bacteria should get to work and can easily be generating large amounts of heat in 3 to 5 days.

Here is a link that might be useful: Compost in 14 days

If my compost pile ever gets above ambient temperature, I'd be surprised. Because of that I am careful not to put weed seeds into the pile. They are likely to come out ready to sprout.

Large amounts of heat? What's large amounts?
Considering the ambient temperature of the air around the pile is still not summer-like, any heat that is generated in apile would certianly air off.
Now there hasn't really been sufficient time to add what makes a pile heat up....grass clippings and such.
There's time galore to see that heat will accompany a pile once it gets going.
And yes, too much moisture does curtail action. Turning it and leaving off so much water should begin the process much better.