iPhone 6S+ not recognized in iTunes

My iPhone 6S+ is not being recognized by iTunes on my MacBook Pro when connecting via USB cable. The phone charges normal, but the square icon does not appear in iTunes, to allow me to sync the phone, add/remove files, etc. Both USB ports have the same result. Also tried on a different MacBook Pro, with the same result (charging, but not recognizing). The iPhone does not pop up the standard "Trust This Computer" when plugging into a new device. My iPad Pro acts normal when connecting via USB to my MacBook...it charges, and displays in iTunes, allowing changes, etc. Mac OS and iOS on all devices are current versions, and USB cable is Apple-original.