Well, it wasn't quite just the motor shield. Because I had mounted some components on the motor shield (which were not wired into anything, yet), I had to use a couple layers of stacking headers to get enough clearance to mount the shield. Then, to provide for some sensors, I put a blank prototyping shield in place. It was the v5 shield, and is missing some critical features the v6 board has, such as a through-ISP connector and eight pins instead of ten pins for one of the sockets (the v5 appears in
https://learn.adafruit.com/adafruit-pro ... ld-arduino, and the v6 appears in
https://www.adafruit.com/product/2077). The goal was to control the motor, read the sensors, and use the touchscreen to display information and provide some touch-control. One of the two pins is marked "IOREF" and the other one is unmarked. I don't know if the absence of these two pins is related to the problem, or the height of the stack is the cause (see photo), but when I plugged this stack into the Arduino UNO and plugged the touchscreen into the top of the stack, it didn't work. When I removed the stack, and plugged the touchscreen directly into the board, it worked.
See the attached photo; sorry about the pins blurring, but the autofocus latched onto the wires (which are in perfect focus); I decided that the pins aren't so blurry as to be confusing or misleading.
So I'm going to have to solder some stacking headers onto the R3, so I can plug this stuff into the bottom, so the stack will be display, Arduino, Proto-shield, and another motor shield with the rotary encoders soldered onto the bottom, as the last element in the stack. Fortunately, the cost of doing this is down at the "pocket change" level. I can spend more than this for a cheeseburger and milkshake. Or half a book. And I buy lots of books [my backlist is now measured in cubic yards].