DIAGNOSIS The locking tabs on the lid or the matching slots on the pitcher's rim are probably worn, broken, or have some debris stuck in them. These tabs are what give you that secure, non-rocking fit.
FIX Take the lid off and turn it upside down. Look at the underside rim—you'll see two or more small plastic tabs or hooks. Now, look at the top rim of the pitcher where the lid attaches. You should see matching slots or grooves. Use a flashlight and a toothpick to clean out any sand, grit, or hard water scale that's packed into those slots on the pitcher. Then, inspect the plastic tabs on the lid. Are any of them cracked, chipped, or bent?
IF THAT DIDN'T WORK If the tabs look fine, the problem might be general wear. The plastic can flex over hundreds of cycles. Try this alignment trick: place the lid on the pitcher and apply even downward pressure while giving it a slight clockwise twist until you feel it drop into place. If it's still irredeemably loose and wobbly, the interlocking mechanism is likely too worn to fix. The lid assembly is a single piece, so you'd need a replacement.
HEADS UP Forcing a misaligned lid can snap those plastic tabs right off. Always make sure the lid is sitting square on the pitcher before you press down. If it feels like it's catching on one side, stop, lift, realign, and try again.