This idea that frequent checking of prices is, in my view, an urban myth (at least as far as agencies are concerned). People like ebookers use a cacheing technique. They have to because the GDS that they deal with don't want the agency constantly asking the same question (it plays hell with their ability to provide a fast response). Indeed, the GDS charge over use fees. And there are similar arrangements in place between the GDS and airlines for the same reasons.
So what happens is that the user asks a question and the agency consults its' cache of availability data. If they haven't renewed the data for a period of time they may well ask the GDS, who in turn will consult their cache and may ask the airline. Since only the airline controls inventory you can see the fact that the question may never even reach the airline cannot affect the number of seats the airline is allocating at a given fare.
In fact, you can see the reverse effect in play when you get offered a fare and then, when you go to commit and the agency finally gets to go to the GDS and airline you can get the dreaded "that fare isn't available, how about this one" (usually more expensive) because they have found out the cahced data is incorrect.
The airlines have all sorts of ways of determining what to charge for a seat. The speed of sale is certainly one but generally they are looking at historic data for the same period (and they can, for instance, tell the difference between an early Easter and a late one and what effect that has on sales) in previous years. They will look at special events such as the Olympics. They consider holidays, business patterns, bank holiday Mondays vs ordinary Mondays. Mondays in March vs Mondays in August.
One critical thing to bear in mind is that it's not just the city pair you are looking for. Hong Kong to Vancouver might well be a high ethnic travel route but if (and I'm making ideas up here) August also happens to be a time when Thais want to visit Vancouver and HK Chinese don't then Cathay may well offer fares BKK-HKG-YVR and set the fares HKG-YVR higher to actively discourage bookings.
Then there are groups. One large group on either half of your round trip can have an effect. If they've taken all the cheaper fares .....
Basically they have all the aces. Good luck.