I Have Prominent Recurring Beau's Lines On My Right Thumbnail

I have prominent recurring beau's lines on my right thumbnail and very mild ones on my left thumbnail and both big toes. I have had blood tests to check my blood sugar and thyroid and they are all normal. Apart from a vitamin D deficiency, which I've been taking supplements for, all of my other vitamin levels appeared normal. What else could be causing this? I've had them for about 8 years and can't find any answers.