Minecraft – why will the hopper not move items to the next hopper

From your description it sounds like you are most concerned with testing the contents of a hopper that is filling a minecart (either a chest minecart or a hopper minecart) and performing an action based on its empty or full state. This is good because it is quite easy to test, as opposed to testing the contents of the cart itself.

Since you mention in a comment that you are basically building off of my previous design, but adding an automatic option to send the minecart instead of pressing a button, I'll show you how to modify it accordingly:

After adding a hopper above the cart, add a comparator which powers a redstone torch. This will turn on when the hopper empties. This signal then powers a rising edge detector circuit that consists of a dropper feeding another hopper, which in turn feeds back into the dropper. Place a single item in the dropper. A comparator tests the contents of the hopper, and a repeater feeds the signal into the powered rail. Whenever the feeding hopper empties, the rail powers and sends the cart along.

There is this one character (I don't know what it is yet, either), that sometimes, somehow, makes its way into commands. Try going to the beginning of your /testfor command, and holding -> until you reach the end. There may be a spot where your cursor stops moving, and there should be a zero-width invisible character there. It shows up in regular editing software, I always check if my commands aren't working.