DIAGNOSIS The silicone seal in the sipper lid isn't seated correctly, or the tab you flip to open the sipper hole isn't fully locking into the "closed" position.
FIX Take the lid completely apart. There's usually a main lid, a silicone seal, and the flip-top sipper piece. Rinse all parts, make sure the silicone seal is perfectly flat and not folded. When reassembling, press the sipper piece down firmly until you hear/feel a distinct *click*. That means the locking tab has engaged.
IF THAT DIDN'T WORK The silicone seal itself might be damaged or have a small tear. Inspect it closely. You can sometimes order replacement sip lids separately if the seal is integral to the lid.
HEADS UP Don't over-tighten the lid onto the cup. It should be snug, but cranking it can warp the plastic or misalign the silicone seal, creating gaps. Just tighten until you feel resistance.