The sign "late" is generally done in one quick motion, but a double motion is
also quite common.
I use the single quick motion if I'm being matter of fact or accusatory.
This sign is similar to "LATE" except
"not-yet" uses a small negative headshake and covers the lower teeth
with the tongue. Yes you have to use your tongue in this sign or you are
doing it wrong. That is another reason why it is not possible to speak
and sign ASL at the same time.
There are a couple versions of "later." This one is sort of like a clock.
The other one does the sign "future" using an "L" handshape.