You are lifting 5-6x/week? I agree with Wendy, you are not giving your muscles time to recover. When we lift, we are causing tiny microscopic tears in our muscle fibers. During recovery days (meaning not lifting using the same muscles 2 days in a row) our muscles heal those tears we created. This is when our muscles grow in size & strength. If you never allow your muscles to recover, you are actually breaking them down and defeating the purpose of lifting. You will not see any gains. I use PS, S&H or MIS (I cycle through every 2 weeks, alternating total body & split). I give my muscle groups at least a day to recover, so I'm lifting 3-4x/week. For instance, I'm doing PS last week & this week. I'll do back/bi's/abs Monday, chest/shoulders/tri's Tuesday, cardio wednesday, legs/abs thurs, cardio friday, and maybe legs/abs again saturday. If it's MIS week, I'll do this total body routine every other day, alternating with cardio. I've seen fantastic gains in size & strength with this rotation (total body/split, recovery days in between muscle groups). Some bodybuilders lift very heavy but only work muscle groups one day a week.
Try to increase your recover days & see what happens. Good luck!