Mobile devices unable to ping iMac on same network

I am running 10.13.1 on a late 2014 i7 27" iMac.

For years I've had no trouble using an app called PhotoSync to move photos bidirectionally between computer and mobile devices. Lately the bidirectional communication has broken down. The iMac can send to phone fine, but neither iPhone 6s+ running iOS 11 or Samsung S8 / Pixel 2 phones running latest versions of Android OS can "see" the iMac to upload photos. I was able to ping mobile devices easily from the iMac. When I use a ping utility on the mobile devices I am unable to ping the computer.

Both wifi and bluetooth are active. All mobile devices are using the wifi network without issue for internet access.

Any suggestions?