It also might be that there is an air pocket in that loop that hasn't been bled. The circulator produces just enough head to move water through the loop if it is totally filled with water. Checking this costs nothing and ought to be done before spending money for parts or a plumber.
Look at the elbows at the ends of the loop. One of them ought to have a small valve in it. Open it until air bleeds out and there is just a squirt of water. You'll want to have a rag under it to absorb the water, and you may have to use an offset screwdriver to get into the restrictive space. And mind your knuckles on the aluminum fins on the pipe, as they are sharp. This may not work if the run of pipe is not level and the bleed valve is at a low point.
If you can't bleed the air out that way, then you may be able to do it through the water makeup valve on the system. Hook up a hose (an old washing machine hose is nice and short) to the drain valve on that loop and run the end into a bucket. Close the valve going from the circulator to the boiler, open the drain valve, and, if necessary, open the pressure regulator bypass valve. The rush of water into the system will quickly flush out any trapped air.
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