You might try blowing on the tube from the outside to confirm that there's an obstruction.
If it is obstructed, 1/4" poly tubing may be small enough and flexible enough to insert into the tube from outside and push up gently into the refrigerator to clear any obstruction. Be sure to keep the end cap and replace it after you're done. The end cap allows water to drain while restricting warm air from flowing back into the refrigerator.
If your drain hose is brittle, it may break while you're trying to clear it. If so, it's time for a new hose anyway.