Yes, reg leaking. Not leaking air out, but allowing high pressure air in when it shouldn't. This builds a higher pressure for the firing valve to open against, it can't do that properly, so the first shot goes low.
After that, so long as you shoot with not too long between shots, the power stays up and stays put.
In a competition, you'll see some shooters ask to fire an empty shot if there has been a hold up or delay between taking the last shot and this one.