You've checked the temperature, picked the appropriate wax, applied it exactly the way you're supposed to, and - WHAT!? no grip. What the heck is going on here?
First off, it helps to understand how grip wax works. Snow crystals have spiky "arms". Wax is relatively soft. We want the "arms" to grab onto the wax when you kick, and to let go when you glide. Simple?
Not so simple. Here are five factors that have a big influence on how much grip you get when you're skiing.
Factor 1: air temperature
This is the temperature that is shown on most wax tins. You check your thermometer (or the Environment Canada website), then pick the wax that is appropriate for that temperature. Air temperature is a major factor in wax choice and is often all you need to consider when choosing your wax.
Picking the right wax can be confusing.
Factor 2: snow temperature
You may have seen avid skiers stick a special thermometer in the snow and ooh and ahh and then pick a particular wax out of their Swix pro waxing cases. They are checking the snow temperature, which lags behind air temperature. In other words, if you go skiing around noon when the air is about -5C, the snow may still be several degrees colder from last night's low.
Factor 3: humidity
When it is very humid out, you are going to need a stickier wax. If there is frost all over the trees and you have to scrape your windshield, it's generally a good idea to go one wax warmer than the one recommended by air temperature alone. (For example if the air temp calls for Green but you know it's humid, go with Blue.)
Factor 4: technique
There's only so much a wax can do if you're not setting it in the snow properly. Remember those little "arms"? It is really important to have a good kick and complete weight transfer so that the wax and the snow have the chance to make good contact with each other.
Look at this guy. He's got some great weight transfer going on.
Factor 5: snow condition
Lastly, but most important - the condition of the snow crystals. In English we have a pathetic vocabulary for describing snow, and most simply we refer to "fresh snow" (snow that has been on the ground less than 24 hours) and "transformed snow" (snow that is older than 24 hours or has been skied on a fair amount). Most wax tins have two temperature ranges: one for fresh snow, one for transformed snow.
When snow crystals first fall, their "arms" are very spiky and can easily penetrate wax. That's why a harder wax can work well. As it ages or wears by melting or abrasion, the spikes round off and become blunt - thus you need a softer wax that is easier to penetrate.
As for ice, which has no "arms", you pretty much need glue. Otherwise known as klister. Best to get out the waxless skis in these conditions.
Kevin's rule of thumb: pick the wax you think is right, or slightly colder than needed. Test it out. If you need more grip, go warmer. Your wax is right if you pick up your ski after skiing and there is a layer of snow on your grip zone that you can easily wipe off with a finger.