Advise from other EC moms

We started EC late but my little man got it down fast.(he is now almost 15 months) He loves his potty, he is not into the big potty yet but I am ok with that.

So when I think he has to go I will do our sign, and if he does have to go then he runs over to his little potty and waits for me to come help him get on.

Other times he will just go to the potty and wait for me to come help. But he will never do the sign.

We use the sign for potty all the time. When he has to go, when he is on the potty, when I am on the potty, when I use the word potty. He does know the sign very well.

So I don't understand why he dose not use the sign to let me know he needs to use the potty? He is in pullups now so if I don't know its time to ask him to go or if I miss him pointing at his little potty then he will just use his diaper.

How can I encourage him to use his sign when he needs to use the potty?