What poses did you try? How did the finished photos look compared to the ones you used to take? Are you seeing a bit of improvement?
Now that you have figured some things out on background selection and had a chance to try out some posing, the next element is lighting. Does your tongue stick out as you move your table lamp all over the place trying to get it to shine just the way you want? Sometimes getting the nails to be highlighted and look as great in the photo as they do in person is frustrating. One thing to understand is that there are some nuances only your eye can register in the same way there are some only the camera can see.
The key is to light the nails without washing them out or creating additional shadows. The most perfect lighting is natural. On the rare occasion you have the time, step outside and use indirect sunlight to give the best effect you can achieve. Next thing to try is more than one light. If you have a table lamp, keep a phone-sized ring light or other portable/stowable light on hand. By shining light on the nails from more than one direction, you eliminate a lot of shadowing which can make colors look dark or sparkles look dull.
Ring lights can create their own problems, leaving circles on the nails that cause them to appear bulky. This is an optical illusion; however, your potential clients are most likely not going to realize that. Angle the ring light so that it creates either a nearly solid line or a very thin elongated oval for the highlights, versus the wide circle. It is not often that flash makes the nails look good; however, always snap one with flash just in case, since it does not really take any extra time. Sometimes it will highlight a sparkle or design in the nail and make the shot pop a bit more than it did with regular lighting.
Right before you take the photo, you should lightly touch the screen of the phone right over your favorite nail or the center nail. The camera on your phone is meant to do a lot of the work for you, so not only is touching the screen going to adjust the focus, it is also going to adjust the camera shutter or lighting which can make all the difference. To experiment, just grab a few bottles of polish, some tools, a knick-knack, or whatever is handy at your table. Take a photo just using point-and-click, then a second time touching the screen to adjust focus and lighting. Flip back and forth between the two or drop them side by side in a collage to see the difference it can make.
Lighting can make amazing nails look average and average nails appear amazing. This is something that is worth your while to focus on and practice. Even after years of taking photos, there are days it never seems to go right and I am always happy to watch others work and pick up some tips in classes.