- Try updating your LR to latest version that is LR CC 2015.2.1
Are you on OS X 10.11 (El Capitan)? A number of people are reporting this problem in the official Adobe feedback forum: LIGHTROOM - Find and Full Screen command key shortcut conflict under OS X 10.11. Please add your vote and description of your issue to that topic, to make it more likely Adobe will prioritize a fix.
Note that upgrading to CC 2015.2.1 has not fixed the issue, at least as reported by several people in that topic. Many of us, including LR authors Victoria Bampton and Laura Shoe, recommend you don't upgrade to 2015.2.1 -- it has serious problems, as even a quick perusal of this forum and the feedback forum will attest. Wait until Adobe cleans up the mess, and let others do the beta testing.
thanks John - yes, I am on El Capitan. I moved my concern to the thread you directed to. After skimming even a few posts it is very clear that the answer is not to move to 2.1.