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?