You don't indicate the source ...

You don't indicate the source of the wastewater, which makes a difference. Do you have an analysis of it prior to these changes? If nothing mechanically has changed, then something might have been introduced that has changed its chemistry. You should also look at the sulfuric acid that you're receiving to ensure that it has not changed. Over what period of time did this occur? The redox will tell you something has changed if you have historical data. Ensure that your pH reading is accurately monitoring the process.