Calculate net force on the pendulum In this image  you can see a swinging pendulum and that there's a net force $A$ that causes it to move in circular motion. It is constantly updating velocity vector $v$ direction to be perpendicular with the direction towards the pivot point.
As I understand the net force $A$ should be $A = T - mg$, where $T$ is the tension force and is equal to $T = mg\cos\theta$. I tried to solve for net force $A$ but I'm not getting the correct result. In my calculations when $\theta = 0$, the net force is zero. But that's wrong because there's clearly always a force that affects the pendulum, since its velocity vector constantly changes its direction.
What am I missing?

 A: You are missing concepts.
In a pendulum, the net force at any instant can be resolved into radial and tangential components(They are infact mutually perpendicular to each other at each instant). The radial component is along the string, directed towards the centre of circular motion. It actually provides the required centripetal acceleration . When the string makes angle $\theta$ from the vertical , you can write:
$$T-mg cos\theta=\frac {mv^2}{l}$$
Note that at the instant of maximum displacement only, the pendulum instantaneously comes to rest. So at maximum angular displacement  $\theta_0$, we can write:
$$T_0=mgcos\theta_0,$$ where $T_0$ is the tension in the string at that instant. 
Note that the tension also varies accordingly throughout the motion. 
The tangential component of the net force is $mgsin\theta$  which provides the required tangential acceleration (or, restoring torque for oscillation)This is along the motion of the pendulum. This is zero only when the string is vertical. 
So now you can see, at the extreme position, only tangential component of force is present. 
You can use mechanical energy conservation for further analysis of pendulum system.
A: A is simply A=-mgsin(theta) 
The tension in the string has no effect on the restoring force, which is dependent only on the mass and the angle to the vertical. 
