They simply lack a terrific tone. Develop a Concept of Tone Production. Listen to several professional clarinet recordings and decide which performer has the kind of tone you want to play with. Keep in mind that every player is unique and that mimicking that tone, while appropriate, will still result in your own unique sound. Use air properly.
Most students use very little air.
They take too little in and blow very little out. G on the staff sounds uncontrolled and out of tune. Take more mouthpiece. It's a fact. The more reed that vibrates, the bigger the sound will be.