It appears this has been an issue since SW2011   I'm using SW2016,  it just stopped working yesterday in assemblies. 

I can get it to work if i spend a ridiculous amount of time to make a simple measurement between parts.  

Work around is to start the tool, Right click, select "SELECT",  click on surface/ edge/ whatever, hold control and select other object,  Then click on the measuring tool dialog box.

Insane!   I use this tool as much as zoom or rotate every day.  What is going on???  I need it to work yesterday, when it stopped!!

Need a real fix ASAP!!!    and reinstalling SW is absurd, it's only been running on this system for 4 mo.