Why a file designed on PC "A" doesn't open correctly on PC "B"

We have 2 designers. Designer A with X7 and WIN10. Designer B (Me) X7 with WIN 7 PRO

Designer A designs a sign and leaves for the day. Sign gets approved. Designer B should be able to open the file and prep for print. NO, when I open it all the fonts default to Arial without any indication as to possible missing fonts. I then have to go over to Designer A's workstation and figure out what the font is go back to my PC and fix, yada, yada, yada.

This slows the whole process and I can't, for the life of me, figure out why this happens on  regular basis.