I have noticed a drop in quality of the Cheddar I have made recently and suspect a change of culture is at fault. They seem soft and bland, lack of acidity?

I have moved from small packets to a larger size and think I have to increase the amount of starter.
If I start the make and find the PH won’t get to a marker, Can I add more culture? Or would I have to wait a while till the PH gets to the marker?

This will be a new approach for me so any constructive help would be appreciated.

I have added culture during the process in the past, however I dont know if it may have been the rennet the problem, old age.

I’m a novice at this pH stuff myself, but here’s a thought:  As long as the culture is even moderately active, it will continue to grow and the acid level will increase.  It may take longer to get going, but bacteria multiply geometrically, so I would just hang in there a wait a bit longer.  I know that with one of my batches, I had to wait nearly an extra hour to get the the proper mark.  I’m not aware of rennet affecting pH - am I wrong on that?

No it slows the coagulation because the rennet is tired. Had 2 batches that did not set because the rennet was old.