After new battery/charge port, still unable to charge.

Hello I'm the lead device repair tech at my local batteries plus bulbs. In the past 3 weeks or so, we've had an unusual amount of iphone 6, 6s, 6+ and 6s+ come in to be repaired for batteries/charge ports. The majority of these phones were unable to be turned on to test the functionality of the device. Along with that, when plugging in the iphones to my volt/amp tester, most of the phones had 0 amperage or 0.01, 0.02, etc. After installing a new battery and new charge port many of these phones were still unable to charge. I would say around 80 percent of the iphones that read 0.00 amps were non-repairable. I asked each person what kind of issues they were having before this happened. Some said their battery was dying faster than normal, some said they hadn't noticed any issues at all. Some customers did mentioned they had recently updated their phone but a lot of the phones that were having this issues hadn't been fully updated anyway. All the phones non-repairable phones were able to turn on with my new battery but even with a new charge port, they still read 0.00 amps. Any ideas?