DIAGNOSIS The little plastic latch tabs on the underside of the lid or the matching slots on the reservoir have worn down, broken, or gotten gunked up with hard water scale or sticky residue.
FIX First, clean it. 1) Wash the lid and the top rim of the reservoir with warm, soapy water and an old toothbrush, paying close attention to the latch mechanism and the slots it clicks into. 2) Rinse and dry thoroughly. Often, built-up grime is the culprit. If it's still not latching, inspect the small, hook-like plastic tabs on the lid. If one is visibly broken or sheared off, the lid needs replacing.
IF THAT DIDN'T WORK The problem might be the reservoir itself. If the plastic around the latch slots is cracked or warped (sometimes from dishwasher heat), the lid has nothing solid to grab onto. You'll need a new reservoir or pitcher.
HEADS UP Close the lid by pressing down on the center, not on the flip-top spout. Pressing on the spout puts uneven pressure on the latches and is the main way they get broken over time.