It's really difficult to say what the cause is for you, without seeing you bowl, however, there are a number of possible issues.
When you deliver the ball are your knees bent or straight? The straighter your legs the harder it is to stay balanced at the line
What are you doing with your non-bowling (left) arm? If you're not putting the left arm out effectively, then it can adversely affect your balance.
What are you doing with your right leg when you reach the line? Where is it laterally relative to your slide foot, where is it relative to your body? The positioning of your right leg determines to some extent how much you can bend your front leg.
How fast is your approach? The faster your approach to the line, the harder it is to stay balanced once you get there.
If all of these are fine, it might be something as simple as leg strength. If your slide leg isn't strong enough then you'll have problems staying balanced on it at the end of your shot
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Reporting from England