Settling difficulty

Good day fam.  I'm at a water treatment plant in Ghana.  We were previously using aluminium sulphate salt for our coagulation process.  Until recently we changed to electrolyte polymer.  I have run flocculation test on a range of dosages. Dosage range of 15ppm - 25ppm work at the lab but when applying on the plant, the flocs don't settle. We even experience carry over at nights.  Any suggestions?