it all depends on the strain and stretch of your phenotype, i usually trim the bottom 10% at the end of week one, two and three into flowering or at least until the plant decides to start her new flower bud stage when you start to see all the hairs appearing. the trick is to envision what nodes...
ive had the same problems and i used a product called sensizyme. the sensi along with the aquasheild should do the trick. i also found another good product called mycogrow. its similar to aquasheild in that it has good mycological spores and good bacteria that plants (specifically the root zone)...