DIAGNOSIS The problem is with the adapter that twists onto the single-serve cup. The plastic locking lugs on the adapter are either worn, broken, or misaligned, so they can't grab the cup's threads.
FIX First, check which adapter you're using. The BN801 comes with two: a larger one for the 24 oz. cup and a smaller one for the 18 oz. cup. Make sure you're using the correct one. Inspect the plastic lugs inside the adapter. Are any of them chipped or cracked? Now, take your single-serve cup and ensure its threading area is clean and dry. Place the adapter on the base, then firmly press the cup down onto it and twist clockwise. You should feel a distinct 'catch' and then it will lock.
IF THAT DIDN'T WORK If the lugs are visibly damaged, the adapter needs to be replaced. You can't fix broken plastic lugs. Also, inspect the cup itself. If the plastic threads on the cup are stripped, the cup is the culprit and needs replacing.
HEADS UP Always ensure the cup and adapter are completely dry before attaching. A little moisture makes the plastic slippery and prevents a good grip. Don't force it if it's not catching—cross-threading will ruin both the adapter and the cup.