Jam at altitude tips?

The Colorado State University Extension has some advice for you on canning times (http://www.ext.colostate.edu/pubs/foodnut/p41.html#can), but I don't think the issue of jam thickening or not is related to altitude. If the advice for the cold-plate test doesn't help you, you might consider ClearJel for both canning and your pies, rather than plain cornstarch (http://www.myspicesage.com/clear-jel-cook-type-p-570.html?cPath=1_46)

I checked my copies of Put 'em Up by sherri Brooks Vinton and The Ball Complete Book of Home Preserving (both excellent resources) and they both agree that for boiling-water processing you should follow these guidelines:
Altitude in feet: 1001-3000 Increase time by 5 minutes
Atitude 3001-6000 Increase time by 10 minutes
Over 6000 Increase time by 15 minutes

I think the one point to jam making I forgot to mention is achieving a certain brix or solids in the final product. This is done by the amount of sugar used and the amount of cooking done on the product . . . but you can over cook jam, which gives it an oxidized look and taste, hence, high altitude equals lower temperature of cooking, which returns to my previous comment of vacuum evaporators (makes a fresher look and taste). I use the old back of the spoon method to see the viscosity of the jam during cooking, since I no longer own a refractometer to check brix (68 degrees brix minimum is considered, by law, jam . . . preserves are 67 degrees). I hope I am not getting to technical. Try increasing the amount of sugar to help with getting enough solids (brix) in your product. It may take a few batches to get the technique down, but that is the fun of learning.

First of all the National Center for Home Food Preservation rocks. (jams and jellies: http://nchfp.uga.edu/how/can7_jam_jelly.html) But doesn't seem to have any specific article on high-altitude canning, it does however tell you how to alter sterilization and processing times for high-altitude and has a chart with processing times with all the recipes.

Personally I've never been able to use the temperature of my jam or jelly to tell when it's done (the two times I did this I ended up with syrup and with candy). I test by either using a chilled plate like louisez does, or I check that it "sheets" off the back of a spoon like the Joy of Cooking suggests.

If you are using corn starch to thicken your pies and sauces the reason they might not thicken properly is because it needs to come up to temperature before thickening. Lower boiling temp might mean that they're not getting hot enough for long enough for the starch to do it's thing. maybe?

I, too, never use pectin. Some recipes call for lemon juice to help jell. I use some recipes like that -- it depends on the fruit type, and some not.

Most commercial jam is made using a vacuum evaporator . . . another words, at sea level, the jam will be cooked under a vacuum so the jam boils at a lower temperature, which can give you a brighter colour and taste. The art of using pectin to achieve the desired set, depends on whether the type and amount. Personally, I never use pectin, but rather macerate the fruit in sugar for 24 hours to help extract the natural pectin. My jam's set is always tender, but holds its form well. If the fruit is too ripe, you will not get a good set.

I live in Montana, in a city that's about a mile high. When I make jam, I test for jell using a plate I've put in the freezer. A tsp. of the jam put on the plate, and allowed to sit about a minute, will not ooze or separate when pressed lightly with a finger. I don't rely on temperature for testing jell. When jam is jelled, I put it in sterilized freezer-proof containers and freeze -- I don't can the jam. Freezing works well, is easy, and jam keeps well. Reaching point when jam jells may take longer than at lower altitudes, but testing for jell takes care of that as an issue. Being able to make jam is one of the best parts of summer -- I hope it will work out well for you. Come winter, it's lovely to retrieve a piece of the summer in the form of fruit you've turned into jam yourself.

My understanding is that to compensate for elevation, you only need to process the jars for longer. In the back of the Ball preserving book there's a chart that shows how much longer depending on your elevation.

I am in the Colorado mountains also. I've never had the courage to attempt making jams or jellies. I do make fruit pies and sauces without difficulty (using the same recipes that I make in Florida without any changes) so am wondering what is your elevation? I'm at 7700 feet.