I'm still a relatively new grower. But having read a lot of articles/journals and threads, I'll contribute whatever I know so far.
You cannot just rely on the light for large yields. There are many factors that may be involved.
1. Heat, is the temperature at the optimum range? If not, that may be stressing your plant.
2. Nutrients, are you feeding them enough for optimum growth? I'm sure you're not overfeeding, otherwise you would know, considering you have grown before.
Is there anything different you did between your first grow and the ones after that? perhaps pot size, different soil, different environment, different light cycle, different strain?
Many of those contribute to the way your plant grows.
I'm going to assume you are growing regular seeds, and not autoflowers. Did you change how long your vegging/flowering stages go for?
Remember, the light bulb does play a big factor on yield and quality of the bud, but there are many other factors that contribute to it as well.
Hope this helps, if there are any experienced growers out there, correct me if I'm wrong.
Good luck!