Syncing midi clock to FL

I'm trying to sync my midi controller (MPK49) to fl so that things like note repeat/arpeggiator work, but having some trouble. I have the controller clock set to external, and I have it selected for output with send master sync selected. What else do I need to change? As a side note, awkwardly enough the very first time I changed output to the mpk49 and checked master sync it worked for about 5 min. then stopped, and I didn't change any settings. I've tried everything to reproduce that, but haven't got it to work again.