iFilter x64 works for one computer, but not another. Why?

Greetings.   I have two Win7 x64 computers.   Both have Acrobat Reader DC (2015) installed, both have iFilter 64 11.0.1 installed.

One is indexing the contents of pdfs just fine, the other is not.   I don't know what the difference is.

On the one that isn't working, I have...

re-installed Reader and iFilter a number of times.

I've rebuilt the index numerous times.

I've verified that PDF Filter is used in the File Types section of Indexing Options in Control panel and that Index Properties and File Contents is set for pdf and pdfxml

I've verified that the settings of what folders to index are the same. 

The only thing I haven't tried yet is that I found a document ​that claims you need to add the iFilter bin directory in C:\Program Files\ to the PATH variable.   But I checked the PC that *is* indexing correctly, and that folder is not in the PATH so I am skeptical that that is necessary.   Even if it was,  Why doesn't the iFilter installer do that?  

I have since added the ifilter path to the PATH variable and I'm currently waiting for the index to rebuild, but as I said, I'm skeptical.

What else is there to check for me to troubleshoot?

Thanks in advance!