I just finished a grow and I have a thread going on about using a 10K bulb and having an issue with smaller plants, despite growing the same strain for decades. I was going to swap it out when I discovered that instead of using the 10K bulb, or the regular 4K MH bulb, I had put a
Hortilux Eye Blue into the socket. I also had issues early in veg with spider mites, so I attributed the shorter plant to the mites, but now I'm suspecting the
Hortilux. It produces about 25% less light than my regular bulbs. I really hate to say that the Eye Blue 1000w bulb was the reason, but so far, that is the only common thread that I have with smaller size plants. I haven't dried or cut them yet, and they are a Sativa so I won't get nugget buds off of them anyhow, but they are smaller than they used to be.
Another issue that I remember from long ago was clone degradation. Despite clones being copies, they seem to decline in vigor over the years and sometimes benefit from newer stock. I love
Promix and have used it since the early 80s, but I haven't seen the decline that you have seen, although I use a very simple process without much added other than compost/manure, wood ashes, and some timed release prills. Jacks 20-20-20 is main veg food, bloom formula for flowering. Same process forever, but the
Hortilux made a difference. If you have a photometer, check your output to make sure it isn't degrading. Is there any chance that your CO2 burner is producing CO? Low levels can cause decreased vigor. A longshot, but something else to think about.