Is pool Chlorine getting weaker?

[Obiwan_Pierogi]

I have been operating my swimming pool at my campground, for the last 37 years.. However, I've noticed in the last 3 or 4 years, the chlorine (granular ca hypochlorite 65%), is becoming useless... C-I-L used to make an excellent chlorine, now I can't get it anymore..
Anyways, for years, when I went to super chlorinate the pool, I'd use 2 full scoops of the powder.. About a litre's worth... Within minutes, this would turn my test water, a dark shade of yellow.. At least 3 ppm... Now... I'm finding I'm putting in a whole bloody bucket (about a hundred bucks worth), and I can barely get 1.5 ppm.. What gives? And no.. the pool isn't dirty.. Trust me.. After this many years I understand how the chemicals work in a pool..
Even if the level dropped to .5 or .75 ppm, about a hundred grams of the granular Chlorine, would bring it back up to 1.5...
I can't help wondering if they've reduced the potency, as they did with most bottles of muriatic acid, in case some idiot thinks he can snort it.. Where can I buy some really good granular pool Chlorine in Canada.. Like the C-I-L product?