DIAGNOSIS The reservoir is likely locked in place by mineral deposits or coffee residue around the seal, or it was pushed on at an angle and is misaligned.
FIX Don't force it. Use a combination of leverage and loosening agents.
1. Grasp the reservoir with both hands, thumbs on the base of the machine. Pull straight up with steady, firm pressure. Do not rock side-to-side.
2. If it won't budge, unplug the machine. Take the entire unit to the sink. Pour a cup of warm (not hot) water over the seam where the reservoir meets the base. This can dissolve sticky residues.
3. Try twisting the reservoir slightly clockwise while pulling up to break the seal.
IF THAT DIDN'T WORK Look for a visible gap on one side. It may be misaligned. Use a thin plastic card (like a credit card) to gently work into the seam around the entire perimeter to break any vacuum or debris lock.
HEADS UP Never use excessive force or tools that can crack the plastic. A cracked reservoir will leak and is not repairable. If it remains stuck, contact Keurig support.