Why does everyone assume that every problem is caused by a deficiency? What about getting too much of something, like selenium, that can cause nail problems? Selenium accumulates over time and this is why my older animals were toxic first.
If you eat a balanced diet, the chances of a deficiency are pretty slim. Especially for selenium. It is high in literally every meat and grain we eat. More is not always better.
Get a selenium level test. I have studied selenium in foods at great length for a couple of years now. Add supplements on top of that and BAM, you could get any one or more of a thousand different problems. Selenium accumulates over time at very low doses. Thresholds thought to be acceptable are just set too high and this has become a chronic toxicity problem in many livestock across the country. So why not humans?