Midi keyboard works only when app is active

I'm on a macbook pro High SIerra, with iRealPro also running.....
The MIDI icon works fine when Musescore is active, but when I go to do something else AT TIMES, I get no midi until I go back to Musescore and reclick the MIDI icon.....but not always.....any ideas on how to keep the midi going while the Musescore app is not the front app?

Not a big deal, but curious...thanks