Maybe this will help. I had the same issue. Was level 75 and hadn't seen a single one plus a friend I usually roll with hasn't had any show up either. So one day I was in an invite only session with out club president. I mentioned I hadn't seen any and he said he sees them all the time. After doing a few missions and screwing around, he tells me "Hey check your map!" . Sure enough, there's an armored car moving down the highway near us. I attack it and walk away with just shy of $10,000. All of a sudden I'm seeing them everywhere. Ever since then I see them, less frequently but they are there.
So the other night I tell my friend that I roll with all the time that I finally got one. He still hasn't seen one. After a little while, I point out one on the map. Sure enough it's there on his too. He attacks it and takes the money. Now they show up all the time for him.
If your not getting any to show up try this.
-Invite only session
-Bring someone that has robbed an armored car.
-Have them point out when they see one.
-Make sure you are the one that attacks it and gets the money.
After that they should show up for you. Also remember that if your in a public session you will see less because other people will probably be jumping on the spawns. Also, when you attack when it sends up a session wide message to everyone, so be careful.
Hope this helps.
This gives me hope! Thank you!