Easiest possible solution is air in the line. At the far right or far left (whichever is the end of the line) there will be a 90° elbows as it turns and goes back into the floor. On that elbow is plug. Turn your heat on so there is demand in the line. Loosen that plug and let the water flow out. It will be warm but shouldn't burn you. Just grab a bowl and towel. When the water flows smoothly with no gaps in flow, you have bleed all the air out.