As the hand enters and extends forwards underwater in front of your head, think about it sinking downwards slightly as you reach forwards. Here's Rebecca Adlington in action doing just that, notice how the hand gets deeper under the surface:
This shouldn't be a forced motion but a gentle sinking action.
The next (seamless) step is to start pressing the water backwards - "the catch" :
Remember not to pause once fully extended - keep the lead hand in constant motion: entering the water, extending (and sinking slightly), bending at the elbow and catching. All joined together in one fluid motion.
It's not an easy skill but that sinking action could be the thing you are missing in your catch technique so give it a go the next time you swim.
Why do this? Many swimmers think they should keep the hand near the surface as they extend but this doesn't account for the fact your body is rotating and so the shoulder is getting deeper in the water. Keep the hand at the same height and you will drop the elbow beneath the hand which will harm your catch and propulsion:
You can also study all of Rebecca Adlington's amazing stroke technique using our unique footage in the Swim Smooth Guru (subscription required):