Sorry, maybe a dumb question, but how exactly are you trying to measure the speakers? Are you only selecting a maximum of two channels at a time through the A16 as well (not just the cables)? If not, it will give you the waiting message because it assumes you are doing more speakers than cables you have (two) hooked up. If you are not doing this, you need to make it so that you are only trying to measure no more than two channels, or else it will just keep trying to send a signal to nothing.
On the A8 at least, before each set of sweeps, you have the option to select 1 through 8 channels to measure at a time. You would do that by just pressing the various channels on the remote to turn them on or off, and it would show in both the UI as well as the channel lights. There's also an option that needs to be enabled that allows you to keep doing various measurements until you are ready to finalize. Otherwise, after each set of measurements, it will act as though that is the PRIR you want to keep. With the A8, you need to select Menu > SPK. and set finalize at end to "NO" until you are ready to finalize.
I don't know what the new modes are called in the A16, but there was no "Async" mode in the A8. The most basic, and most versatile was called "ALL", which allowed you to capture all speakers at once, or one at a time (or any combination up to 7.1).