Ronald,
I too work with Alumilite and have a ton of experience with it. Most of the time when I talk with folks who have problems with the resin curing it was because they did not get it mixed thoroughly or the measured properly.
Just a refresher: You need to measure by weight, not volume and you need to stir very thoroughly. People new to Alumilite tend to rush the stirring since they are worried about it setting up on them. DO NOT rush this part! You have plenty of time. When you first pour the parts together, they will look kinda cloudy when you look into the mixing container. Stir and keep stirring until the resin become completely clear again. While stirring, be sure you scrape the sides of your container.
If you have stirred and measured properly the only other thing that could be the issue is the temp of your shop. I have never had an issue but my shop never gets below 65 degrees. One thing Alumilite does recommend is that you heat your mold as ElMostro said. Just throw it in a toaster oven and get it nice and warm. This will solve the problem! BTW, humidity does not have any noticeable effect on Alumilite.
Good luck and let us know what you find out. If you need more help, feel free to contact me by PM and I will send you my telephone number so we can talk in "person".