There are a number of factors that could have contributed to this experience: mindset, dosage, efficiency of smoking method etc...
IME, lower dosage experiences can be unsatisfying, confusing, intrusive, and more on the darker side. It's as if you're swimming the outskirts, where the lower forms/entities hangout. The visions are usually dull but quick, and I'm left with an uncomfortable feeling of confusion. I've experienced this many times, so typically I do my best to make sure a breakthrough happens when I smoke.
That breakthrough experience can be such a state of bliss...
I think if you hit it right on the button with dosage, there's no way your mind can fight it and you're launched past this dimly lit outlying realm.
Additionally, I had a batch of changa (1:1:1) that was maybe 7 months old that I just could not seem to break through with after a while. When I first used it, it was extremely powerful and the experiences produced were incredible. I decided to let it go and start with a fresh batch after a couple of failed attempts and my intuition telling me that it had gone 'sour'. The attempts were like the above described lower dosage experiences and
could have been due to technique, ROA, efficiency, and mindset, but something during my trips told me that the batch had gone
bad...
Anyways... someone here more educated than myself might be able to tell you more about DMT's loss of potency over time, but I've never had any issues except for that one time. Hope this helps!
Peace
-monkey
ॐ असतो मा सद्गमय ।
तमसो मा ज्योतिर्गमय ।
मृत्योर्मा अमृतं गमय ।