Obviously, video is your friend, but one mistake that many people make is to dip too quickly. Don't out-run the barbell down in the desire to use extra speed for an extra-heavy weight. You have to go down at exactly the same quick but controlled pace you would use for a lighter weight, and be confident that the speed from your fast reversal of direction at the bottom of the dip and fast acceleration will suffice to make the lift.
Also, make sure that you are dipping by bending the knees, not pushing the butt back as you would in a squat.
Also, make sure that you aren't cutting off the drive in a hurry to dive under the bar. Patience, grasshopper.
Also, you could just be weak. How much do you press/ push-press/ squat?
Finally, you could just be temporarily klutzy. This stuff takes time.