DIAGNOSIS The plastic adapter that stays on your faucet has likely become cross-threaded or has a piece of debris stuck in the threads.
FIX First, unscrew the entire adapter from your faucet spout. Inspect the plastic threads on both the adapter and the filter head. Look for any plastic shavings, sand, or scale buildup. Clean them with an old toothbrush. Now, re-thread the adapter onto your faucet by hand, going *slowly* and counter-clockwise first until you feel the threads drop into place, then tighten clockwise. Then try attaching the filter head.
IF THAT DIDN'T WORK The plastic threads on either the adapter or the filter head might be stripped. Compare them visually. If they look worn or flattened, that part needs replacing. You can order a new faucet adapter kit from PUR.
HEADS UP Never force it. The plastic used is durable but can strip if you muscle it. Always start threading by hand and never use pliers on the plastic housing—only on the metal collar of the faucet adapter if you must.