On start up I can not enter my password because the keyboard is not discoverable. I have put brand new batteries in the keyboard.
And the best tip when using Bluetooth is always have a wired keyboard and mouse on hand for emergencies, installing new software, running some programs such as DiskWarrior etc.
Nowadays it's hard to find an inexpensive wired keyboard. I've had better luck finding inexpensive wired rodents at places like "Big Lots" (discount store), Wal-Mart, and sometimes at Staples. Wired keyboards are becoming rare. I keep two Apple wired keyboards on hand. One has the old firmware (vintage 2008) and a newer one I just bought at our local Navy Exchange at the Naval Base nearby.