The big bang happens and region A and B are expanding away from each other. It takes time to send a signal from A to B due to the finite speed of light and the expansion of the universe, and if we have a cosmological model we can calculate how long it takes.
We live in region C. We get signals from both A and B and since it takes time for the signals to reach us we are really observing A and B at a specific time in the past (which we can calculate). We can see that at that time both A and B are in thermal equilibrium which can only happen if A and B can interact (i.e. send signals to each other). But the problem is that at the time we observe A and B, it is still too early for a signal from A to reach B.
The solution is believed to be our cosmological model was wrong (we need to incorporate inflate) which means it actually takes less time than we originally thought for a signal from A to first reach B.
