Same account different search results for different users

We have recently started using office 365 for our business. We have 6 employees using the sharepoint  system daily for our filing system. 

On multiple occasions a file will not be visible to some of us but is visible to others.  This is creating chaos as we are assuming that the work was not completed or has been lost. 

We can't trust that this system is giving us the proper results. 

It also jumps from Home/Documents or Shared with us regularly on its own. 

Any guidance as to the why this might be happening would be greatly appreciated.