Ports have a limited amount of supply they can give. If you toggle supply on and you see a bunch of "!" then that means you exhausted the port supply capacity.

The HQ increases the about of effectiveness the unit recovers and the range of potential supply.

If there is not enough supply for all the units they get basic supply which is food and water.

Think of a tiny port supplying 8 corps. You just can't do it. All the fuel, ammo just can't be shipped to the front lines fast enough.

As for reinforcements. You have to set the about in the top section of the economy screen. How much you want to spend on it. There is a priority of which units get reinforcements first. You can give units priority to get reinforcements with the toggle on the unit. Or selectively say don't reinforce or upgrade at all.

If you need a tutorial this is a good one.
https://www.youtube.com/watch?v=M_L6W5dZZmc