Two things you could try:
1. Drop your thumb of your left hand down behind the neck. This will help you squeeze your thumb and index finger together to make the barre chord stronger.
2. Use the side of your index finger to make the barre chord instead of the flat part of your finger. If you use the flat part of your finger, you’re naturally working against the joints in your finger which makes it hard to get a good sound. If you use the side of your finger, the bend in your joints won’t affect the sound and you’ll get clearer notes.