Ring3 replacing working doorbell doesn't detect hardwired

I'm installing a Ring Doorbell 3 in place of the doorbell/intercom at my front door and haven't gotten the Ring3 to detect a hardwired configuration. I purchased a new "24VAC, 40 VA AC Transformer Plug in with PTC Fuse Compatible with Ring Nest Doorbell Thermostats UL Certified" to replace the 25 year old transformer that has been powering the intercom/doorbell since the home was built. But, the Ring3 remained operating on battery instead of detecting a hardwired configuration.After trying a few things to isolate the problem (none of which worked), I connected the Ring Doorbell 3 directly to the transformer (plug in wall kind that provides power to intercom/doorbell system) and THAT worked. But, I want the Ring3 to drive the existing doorbell function that's in the intercom system. Do I need to do something else since there are three intercom units on the same low-voltage circuit as the Ring3? Or, perhaps the 25 year old low voltage wiring in my house works for the old doorbell, but the power through it drops off enough that it won't power the Ring Doorbell?Any suggestions?