DIAGNOSIS You've got a complete airlock. For water to flow down through the filter, air needs to escape out the top. If the vent holes or channels in the lid are blocked, air can't get out, so water can't get in.
FIX The main culprits are the small vent holes on the underside of the filter lid, near where the filter sits. Take the lid and filter off. Look for one or two tiny pinholes in the plastic. Use a straightened paperclip or a pin to *gently* clear any blockage. Don't force it or drill it out—just clear any visible lint or scale. Next, check the filter itself. Is it seated all the way? A filter that's not fully clicked into place can cover those vent holes. Reinstall the filter with a firm push until it clicks.
IF THAT DIDN'T WORK There's a less common issue where the one-way valve in the filter (a small rubber flap inside) gets stuck shut. Try removing the filter, shaking it vigorously, and then reinstalling it. If the problem started immediately after installing a new filter, that specific filter might be defective. Try a different one.
HEADS UP Never overfill the top reservoir. The water level needs to stay *below* the rim to allow an air gap at the top for the venting system to work. Filling it right to the brim will guarantee an airlock every time.