2 things it could be aidanh
If you have got to the end of the serations on the strap without the buckle on it you won't be able to tighten it further and need to use the other hole in the end of the strap (the bolted end ) to shorten it
Sometimes it is possible to push the serated strap through the buckle at the wrong angle and it goes over the button that you release the buckle with then they won't tighten properly
If all else fails Pro-jump sell the buckles very cheap