when i started sea of green, same thing happened to me. the inconsistent yields where because of the size of plants while switching to 12/12. depending on how much vertical height ur working with, flower your plants a inch or two taller than the last time you flowered them.

if youve kept the nutes and everything else the same, the only other thing i can think of is the effect of the 600hps bulb, if its new, no need to worry.