Why does my physics2D script cause the player to randomly stop moving?

In the script, it checks player input for a constant key during fixedupdate, then checks if the players horizontal speed is greater than his maximum set speed. if it isnt, then force is added to move them in a direction.

Now, while playtesting, the player has a tendency to accellerate evenly to max speed, keep an even-ish pace for a while, then will suddenly stop moving, and the key will need to be released and pressed again to move again. (function used is Input.getKey(Keycode.A); i believe)

The original script had an else statement, which, if the velocity was GREATER than max speed, it became max speed, but i was hoping to implement a knockback feature in the future, and im worried this would restrict that.

TL;DR: How do you keep a steady movement on a rigidbody2D that doesn't skip out, or move into walls as translate would?