Certain screenshots not recognized when passed between testers

We have a team of four QA testers authoring our Sikuli scripts.

In general, we have no problem passing the scripts from one tester to another- the scripts authored by one user work on another's computer, the screenshots taken by one person work for everyone else.

We ARE running into challenges with screenshots of our dropdown menus, though. The patterns are very basic- one line of copy in a dropdown- or more complex- screenshot of the entire dropdown, with the desired item targeted with Target Offset.

With both the simple and complex versions of the patterns, the click(dropdown_screenshot) commands work fine on our local computers, BUT the patterns are unrecognized on each other's (matching tolerances need to be dropped to 30-50% to get a match).

Additional info:
- Some testers snapped their dropdown selection screenshots directly from the webpage
- Some testers first captured the dropdown content with Snaggit, and then Sikuli-screenshot the Snaggit image
- All dropdown selection screenshots retained their default image name
- All monitor resolutions and sizes are the same
- If one tester pastes their dropdown selection screenshot into another user's script, the script passes as expected
- All other screenshots are working fine across users

Thanks for any insight or advice! Not sure if this is a defect, or user error, or what- though user error on our behalf is the most likely culprit :)