If the tap is a mixer it might be an airlock or a dodgy washer. With a mixer tap it's sometimes useful to put your hand over the spout to block the flow and turn both the taps on, it can help to re-balance airlocks and might dislodge a sticking washer. I have dismantled taps to find badly broken down washers inside but with mixer taps each one is a special mystery created by god to annoy tradesmen.
If the tap is a mixer it might be an airlock or a dodgy washer. With a mixer tap it's sometimes useful to put your hand over the spout to block the flow and turn both the taps on, it can help to re-balance airlocks and might dislodge a sticking washer. I have dismantled taps to find badly broken down washers inside but with mixer taps each one is a special mystery created by god to annoy tradesmen.