Hello Hitch,
As a long time user of drugs (unfortunately and reluctantly) for an anxiety disorder and for blood pressure, I have had to change drugs a number of times when one failed to work as before, of when a particular brand of generic had particularly negative side effects.
For example, Xanax (alprazolam) used to give me a nice euphoria for a while and calm my anxiety quickly. A wonder drug? As I have been changed to two different generics, it now gives me NO pleasant high (in fact, now a headache and unpleasant drowsiness, but it still stops panic attacks).
Also, I used to eat tons of oranges and lemons, and now they're like pouring sulfuric acid into my blood. No change from name-brand to generic here, though.
In answering your question, there are two factors that may be at work:
1. The body changes, adapts, alters its response to the substance. In many cases, like my reaction to citrus, it begins to reject the substance (as if it gets tired of having to deal with it). Ask any allergist - overconsumption of any food runs the risk of becoming allergic to that very food. Also, the age of the consumer has a lot to do with how they react to medicines, as older metabolisms do not process things in the same way as younger bodies.
2. The excipients (extra ingredients) in name-brand drugs and generic forms are often quite different. Most amlodipine (formerly Norvasc by US Pfizer) is generic now, most made by Lupin, a company in India, a country ridden with pollution and disease - I just watched a documentary on PBS that showed how the very water in India often carries an incurable bacteria (NDM-1). Although the main anti-hypertensive ingredient is still there, other different excipients (and water supply) may be used - and they may trigger negative reactions.
I hope this clarifies the issue a bit for you. It is complex and may have various causes, but it is not at all unreasonable that side effects may occur years after starting a medication.
I suppose that I was fortunate in that I began to experience negative side effects (perhaps to the basic amlodipine itself) only a couple of months after starting what I thought was the perfect answer to my blood pressure problems. The side effects apparently "took off" once it had reached a certain level of concentration in my body.
Since it has such a long half-life (persistence in the body), I am still experiencing hot flashes, high BP spikes, racing pulse, etc. - things that never occurred before I took Lupin amlodipine - the Indian generic, which also concerns me.