Trouble training to "speak"

When I got Prince a year ago he already knew some commands and I taught him others (I'm not sure if some, like "dance", would be done without a treat yet).

Over the past month or so I've been trying to teach him "speak". If he (sometimes just thinks he) hears something or wants attention, he'll bark like any dog does but if I tell him to speak (and over the past week I've started adding the hand signal, wagging finger), he'll first look away then at further "speak" prompting, he'll LAY DOWN!
Is he embarrassed by the speak command?
Is he just being stubborn?
How can I break him of this unwanted response and start "speak"ing?
A couple nights I even tried playing (various) barking sounds when I said "speak" and he still wouldn't do it on command. It's like the command puts on the brakes if he's thinking of responding to those barks.