Certainly the mawashi geri (round kick or round-house kick)
is a difficult kick and many karate-ka do it rather poorly.

So what can you do to improve your mawashi geri?

1. Make sure that you are physically flexible enough to execute the kick. Mawashi geri requires that the adductors of the leg (the muscles on the inside of your legs) separate enough to allow the angle between the legs to be greater than 90 degrees. You can check this by sitting on the floor facing a wall and separating your legs like doing the splits against the wall. If you get 90 degrees or more you probably can do a round kick. If you have less than 90 degrees your kick will be low. The farther you can easily separate your legs in this splits-like position the more likely you can do a good round kick. But still you must learn the correct technique.
2. Set up a stretching program to include the adductors splits as part of your routine. Separate your legs against the wall until you get to the point of feeling the stretch (but not pain) and hold it for 15 seconds to 2 minutes. Then relax. For more rapid progress do this two to three times in a session. Don't forget to lean backwards some of the time and to lean forward some of the time. Performing this stretch four to six times per week will lead to good stretching of the adductor group of muscles.
3. When you perform a mawashi geri allow the foot on the ground to rotate (usually on the toes) so that the toes point between 90 and 180 degrees from the enbusen. This allows a greater reach on the kick and allows the hips to play their important role. Often I see students attempting to keep the toes of the foot on the ground pointing forward during the mawashi geri. This prevents the hip action and the extension of the leg forward.
4. Proper hand action is essential to the ability to reverse the mawashi geri and get the foot back to the ground without dropping it straight downward. The arm action is difficult to explain in words. Ask a person in the dojo about the arm action and practice it. The proper arm actions during a mawashi geri will improve the kick immensely.
5. The head does not come forward during a mawashi geri. It may bend to the side (90 degrees from the enbusen) or it may go backwards (greater than 90 degrees from the enbusen). It does not go forward (less than 90 degrees from the enbusen). If the head goes forward it keeps the hips back, and prevents the application of centralization in the kick.

If these hints don't seem to help in your private practice then I suggest that you ask your Sensei to watch your mawashi geri. It is really difficult to write all the possible permutations of errors that students have created for their mawashi geri.
Good luck!