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.
Should have said to open the taps gently enough that you can redirect the pressure back into the spout. I've used this technique successfully a few times but I don't really understand why it works.
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.