I'm guessing at the problem but as I recall from my first attempts, you may need to "condition" the reed a bit by gently pressing in on the edge to shape it into an eye shape. Be very careful not to over curve it. You may also need to soak the reeds in water for an hour or so if you are using wood reeds. Another reason could be you are blowing too hard. As the reed gets conditioned it will be much easier.

Another un-related tip would be to find a bag of Orthodontic rubber bands like are used on Braces for the teeth (not sure what they are called in England ) These are great for making subtle changes inthe pitch of the reed and extending the life of your practice reeds.

Hope these help. If not tell me a bit more about the chanter and yor technique and I may be able to pin point something else. Also check out the college of Piping if you haven't already. They have some great books to help you out.