2 Things.
Big Bloom is not enough by itself to sustain your plant. It may have done ok in the beginning as it used the natural nutes up in the soil as well. Big Bloom is used in conjunction with A vegging nute (Grow Big 6-4-4) during vegatative growth and with a flowering nute (Tiger Bloom 2-8-4) during flowering.
Big Bloom has a fractional N-P-K value. They are starving to death. Due to lack of mutrients and compounded by possible pH issues.
See the Fox farm Schedule and read the print at the bottom. Big Bloom... in conjunction with ....
2nd.
get a good digital pH meter. This is extremely important for a successful grow. Don't get a litmus style tester, or a soil test kit. get a digital meter. 20 bucks on ebay check it out.