How do you remove old dog poop stains, some are hard little pieces and stuck on a carpet I have a new BISSELL machine and the carpet cleaner, it still will not come up...

Answer: For poop stains, generously and evenly spray the area with a BISSELL enzyme action product such as Pet Stain & Odor Pre-treat for Carpet & Upholstery.  It has enzymes that go deep into the carpet and "eats" away the stain.  Per instructions, spray the stain, then cover with a damp towel for up to 72 hours to allow it to loosen up the stain. Then, use your BISSELL upright or portable deep cleaner over the affected area.
If any stain remains treat it with OxyPro® and let stand overnight.