There could be a couple of reasons why you feel that you might not be able to do Reiki correctly:
You may simply need more supervised Reiki practice. If this is the case, do your best to attend the Reiki practice meetings (sometimes referred to as Reiki shares) hosted by your Reiki teacher. Repeating the training is also a great idea. (I personally, take a refresher course at least once a year, and in spite of the fact that I am already a Reiki teacher, I derive tremendous benefit from it.)
You may have learned Reiki in a style that has been modified from the original to the point of being purhaps less effective*, especially if you’ve learned Reiki online or through a quick course without enough hands-on practice. In that case, I would encourage you to find another Reiki class.
You may be overthinking it. Giving Reiki is a very simple and natural process, and sometimes all we have to do is let go of the idea that we are ‘healing’ someone, and allow Reiki to work on its own, so to speak. When we come from a deeper place, and let go of our regular ‘conscious, ego-based mind’ approach, Reiki often works much better. So sometimes all we have to do is ‘let it happen’. (When you experience it, you will know exactly what I mean.)
* Reiki was originally developed in Japan, and spread in the West by Mrs. Takata. For a long time it was believed that there was no one left in Japan who knew and practiced Reiki, and most of the Reiki classes today come from Mrs. Takata’s lineage. Unfortunately, since there was no communication between the Japanese Reiki practitioners and their Western counterparts, some of the newer Reiki styles have been progressively modified to fit the Western mindset, and a lot of information and important aspects of Reiki teaching and practice have been either skipped, lost in translation, mixed with other healing modalities, or diluted, in some cases to a significant degree. Reiki is Reiki, so it doesn’t mean that if you learn a Westernized version of Reiki that it is not going to work, but if you are noticing that you are not feeling 100% about your Reiki ability, then going to the source and learning traditional Reiki the way it was originally taught and practiced, may be the answer. As it turns out, Reiki was actually preserved in Japan in its original tradition all along, with the authentic Japanese style Reiki currently taught by the Jikiden Reiki Institute. I did an extensive research in different Reiki styles, and based on that extensive research, it appears that Jikiden Reiki is the closest to the original tradition out of all the Reiki styles that are available for us to learn today. I would highly recommend Jikiden Reiki (it is incredible that it is now so easily accessible to us!) if you are looking for an effective Reiki technique in its original form for your own wellness as well as to help others.