DIAGNOSIS Mineral scale or coffee residue has built up on the reservoir's guide rails and the O-ring seal, causing it to bind.
FIX Remove the reservoir. Clean the entire exterior with a damp cloth. Pay special attention to the two vertical plastic guide rails on the back. Clean the corresponding slots inside the machine with a cotton swab and vinegar. Apply a very thin film of food-grade silicone grease to the guide rails and the O-ring.
IF THAT DIDN'T WORK The plastic housing of the machine may have warped slightly from heat, misaligning the guides. Inspect for any visible cracks or deformation. If warped, the reservoir will never slide smoothly.
HEADS UP Never force the reservoir. You can crack the plastic mounting tabs inside the machine. This breaks the water inlet seal, causing a major leak. Gentle cleaning is the first step.