I speak confidently and fluently, but my score is around Band 6–6.5. Why?
Confidence and smooth delivery are useful, but IELTS Speaking scores four equally weighted criteria: Fluency and Coherence, Lexical Resource, Grammatical Range and Accuracy, and Pronunciation. You may be able to speak without hesitation while still repeating basic vocabulary, making frequent grammatical errors, giving poorly developed answers or being difficult to understand at times.
Record answers to Parts 1, 2 and 3 and ask for specific examples of weaknesses. Work on the criterion that actually limits your performance instead of simply trying to speak faster or for longer.