Kerien,
There are only three possible explanations for your inability to “experience”
fatigue: Extremely tough ligs/tissue, a super high tolerance for pain, or a flaw in your routine. The answer can also be a combination of any of those three things.
But since we have no real way of knowing, the only thing to do is consider your routine and what might be missing. I can only relate my personal experience with fatigue, and because I have only felt it in relation to hanging I’ll stick with that subject.
I think what you need to find is that place where the formula: Time (set length) + Tension (weight amount) = Growth (fatigue) is most efficiently balanced.
There is a “time/weight zone” you have to find that places you right in between ease and discomfort. You should only be able to hang in this zone for a period of time at a certain weight before you have to leave it. You will then move a step down into a second zone that will ideally share the “time” element of the equation but a little less of the weight. This second “time/weight zone” should also demand that you spend a limited amount of time there - and force you into another such zone (same amount of time/less weight), and so on.
If you are hanging 5-6 fifteen minute sets at fifteen pounds I have to assume you do those sets in good comfort. Whatever your reluctance to move up to 20 lbs you might want to consider trying it. If you find you cannot handle it for the 15 minutes you seem to prefer, you have discovered that somewhere between 15 and 20 lbs at 15 minutes there exists for you that first “time/weight zone”.
By experimenting with weight in that range as well as set length (btwn 15 and 20 mins) you would find the time/weight balance that is going to set you up for fatigue.
To my way of thinking, the most important feature in recognizing fatigue, is hanging with the absence of it. The only way you can be certain that you have arrived at fatigue is to have it appear at a time/weight that is otherwise comfortable save for the mild, unmistakable “presence” of the weight itself. This way, when fatigue comes you cannot mistake it for anything else.
The tricky thing is finding a “time/weight zone” that lives between ease and discomfort; you are hanging comfortably, with the absence of pain, and at a point your body tells you that this is no longer comfortable, it is time to move down to the second zone.
And because it will always vary man to man you have to find that place yourself and use it sensibly.
Please keep in mind, though that you very well may have either extremely tough internal tissue or have an unusually high threshold for pain. So please, do not become overzealous in your quest for fatigue. Always keep in mind those warning signs that you are doing damage to your unit.
People do gain without feeling fatigue, but nobody ever gains by injuring themselves.
I hope this helps.
-Cap