The Wifi shield covers the SPI pins on the ICSP header of the Zero board. It seems that the shield does not bring the SPI pins out to the shield headers. Is there a way to access the SPI pins on the shield?
You could replace the ICSP Header with a stacking 2x3 header, but putting a shield on top of the WiFi Shield 101 is likely to mess up the signal pattern.
It might be better to stack a shield that does passthru under your WiFi Shield. One possibility is the Adafruit Proto Shield #2077.