A tendon blockage isn’t normally something you can fix quickly or while grouting is underway that day. If there is a blockage, some degree of investigation will be needed to locate the blockage and the resulting voids. This can be fairly complicated depending on the tendon geometry. Ideally, use of a “rabbit” pulled thru the duct before inserting the strands will locate any duct restrictions and the air test just before grouting will make sure you have some flow – but maybe not enough flow for grout. Locating the blockage/voids may require a GPR or similar NDE devices. I suspect blockages are more common when you have multiple parallel ducts and there is some “communication” between the duct being grouted and an adjacent duct – this should also be caught by the air test.